(I might have been using poor language with Jorin's dialog.  He was trying to ask why the citadel was sending scouts into the underdark if they know its dangerous, especially having sent several parties down in row that were all lost)

"This is a sensitive situation. Adventurers can be boastful about their deeds and the dangers they face, especially in an ale hall, so I am asking for discretion in your conversations. The scouts are all Lord's Alliance. The non-military folk working around the Citadel, like the townsfolk and the goblins, are friendly but not devoted to the Alliance."

Ardragon gets an official looking parchment, fills in a few blank lines, then takes his stamp and uses it at the bottom.
"This is a writ of authority, to let all know they should cooperate while you are doing the mission. If you have need of supplies, show this to the shopkeepers, and they will get you the items at cost." (PHB prices. Normally, goods this far in the wild would have a 25% markup.)

"If you brought any steeds that you do not wish to take underground, they will be given space and feed in the stables.

"Go past the stables, forge, and barracks, to the fortified mountain entrance. The scouts will be at breakfast in the main dining hall of that building.

"Make what preparations you need for the success of the mission, but do not delay. The trail of the last scouting group grows colder, and if they are in trouble, time will not help their survival."

(EDIT: Just saw Guiltysparc's post. There are hundreds of miles of tunnels under the Citadel and the mountains, so part of securing the region requires mapping and exploring the nearby underground regions.

Not all scout teams sent into the Underdark went missing. After the 1st and 2nd scouting teams went missing, it is normal for people to make a search for those who are lost. A 3rd team was sent to find them. When they also went missing, a well armed 4th group with an experienced leader was sent in to find the others, and surprisingly they haven't come back... There is definitely some out-of-the-ordinary danger lurking below that wasn't there before. No one else is being sent in until the Road Ender's come back.)

(ah ok, so that's pretty straightforward then)

Jorin thanks the man for the writ, and his face lights up at the mention of breakfast.

"Let us speak with the scouts before heading into the tunnels, and get some breakfast ale as well."

Quillary is content to go along with Jorin. She does not move to take the writ, assuming Hildi will take it like he takes all the papers the group comes across.

Spar glances at the writ but is fine to have Hildigrim take possession of it. She nods to Ardragon after he finishes his explanation and has one more question for him. "We'll try our best to get your people back, if they're still alive. This is not my first time underground, but is there anything that successful people bring into Upperdark... or that unsuccessful people forget to bring? I'm asking you what you'd say we should make sure we have with us."

Once she has her answer, she's happy to move along and stop by any shops that make sense to supply up (per Ardragon's recommendations, per the party's predilections, or per whatever delicious food she sees in food stalls along the way.

Ardragon repeats what he already said: he hasn't been in the Underdark personally, so he doesn't have any advice that would be better than what you can get from the scouts who experienced it firsthand. Between the writ, and knowing that the lives of their fellow scouts are in the balance, you can get the answers you seek from them.

The party leaves the Administrative Building. You walk past a cluster of barns that stable different animals, and a pair of barracks with Alliance soldiers stepping in and out.

Jorin hesitates briefly outside the forge, which is active (smoke from the chimney and metal hammering sounds) despite lacking a woodpile or a source of coal for a fire.
But the thought of breakfast ale (and Spar wanting to get more to snack on) overrides his blacksmith's curiosity.

The entrance tunnel to the mountain is the most heavily fortified area in the Citadel and the rest of the fortress has not skimped on defenses. A series of 4 portcullis and 2 iron bound doors can be used to block the tunnel, which has arrow slits on both sides as well as murder holes in the ceiling. Guards move to block your entrance, but holding up the Writ causes them to step aside and allow your group inside.

The underground portion of the Citadel feels just as militaristic as the surface, if not more so. Lit surreally by unceasing, glowing red crystals that seem to have grown in lines along the walls, it seems to increase your comfort down here and the ease with which your eyes adjust. Areas under construction have variable lighting by lanterns and torches. In those areas, the walls are rough stone, the floors are mud, and the ceilings are either natural or supported by wooden beams. The air is surprisingly fresh, and when you stand still, you can feel a small draft.

You pass multiple barracks rooms, armories, an apothecary, and with them, the associated debris of living in close quarters underground. You are pretty sure you saw something that looked like a stable, and was that a farm? Uniformed Lord's Alliance are everywhere, and running around between them are children, some are the more common surface races and some are goblin. You see the occasion adult goblin keeping an eye on things as well, though they do not wear military uniforms.

There are also a number of dining halls. You ask the nearest soldier about where to find the scouts who visited the Underdark in the past, and you are pointed to the main dining hall.

(If you are headed straight to the scouts in the dining hall, then start listing your questions for them.
Specific questions will get more specific answers.  Using small talk will also make them more generous in response.
If you wanted to visit other areas first, then mention what you had in mind.)

Hildigrim happily accepts the writ, smiling at the feel of the parchment before tucking it away. Intending to hurry to breakfast, he pauses to hear what Ardragon might say in reply to Spar.

Quillary thrills a little at passing into the dark, and as she notices the freshness of the air (she’s very perceptive) she lifts a hand to let some spores trail in the draft, noting its direction.

She smiles at the children, the goblins, the dour soldiers. She stoops to feel the dirt at her feet, and at one point kicks off her boots to pad along barefooted the rest of the way. Her eyes track to the stables and the farm, but she does not linger, live animals and green things have never been her love.

As the crew arrives in the dining hall, she follows the others to the scouts, giving them a warm smile and a thorough looking over.

Insight: 18 

Quillary watches a few spores, like smoke, drift over to a vent six inches long and only an inch wide.
After finding one, she notices many, randomly placed around the rooms and passages, that bring air from the surface.
(Not a good design if this place was ever flooded, but you are in the mountains.)

The Lord's Alliance scouts are initially reluctant to discuss their business with outsiders.
They are gloomy as they don't have assignments, due to the Underdark being off-limits, and they aren't able to help their missing comrades.
After showing the writ and stating that you will be conducting a rescue, they become animated and ready to answer your questions.

"From what we've been told, it sounds like you ran into a thresher down there. I'll admit it's impressive you made it back. Anything you want to tell us about? They're throwing us at it, and we'd like to find your friends and not die in the process," Spar says, leaning against the wall. She nibbles at some jerky. 

She listens attentively eyes pinging to watch whoever's speaking (and, seemingly, other areas of the dining hall). She's initially just as weird as she was meeting the party but there's really something compelling and attention-drawing about her. People seem to forget themselves when they're around her.

Persuasion? 

“To study it,” Quillary adds. “Because we are visiting scholars.”

Hildigrim smiles. "So, how does a hungry halfling go about getting something to eat?"

 As if on cue, Jorin appears with a tray of breakfast ale and sundry viddles, enough for the party and also the group of scouts.  He had made a b-line for the serving area as soon as they got there.

He passes the cups around and adds to Quillary's sentence, "Yes, scholars!  To study how my axe falls into things, " and laughs before taking a drink from his cup.

Quillary passes the food and drink along. “I ate before,” she says, pleasantly.

“Tell us, what is it a well prepared group takes with them into the underdark? What are the usual dangers? What do you hope you never come across?”

The scouts are slightly less enthusiastic after the party has spoken.
(OOC: Their first reaction to your party talking about breakfast and a scholarly visit is like Ripley's initial reaction in "Aliens", when the marines don't seem to be taking the threat seriously.)

After a moment to collect themselves, they brighten up again and try to be helpful.
They do slide over some extra mugs and plates with mess-hall food. Quite disappointing after what Pascal prepared before you left.

Drusk points out that the Road Ender's have moved through the Undermountain beneath Waterdeep.
When Quillary asks if there are other preparations to be made, the scouts banter amongst themselves.
"Since you are already familiar with traveling in monster-filled caverns, you probably have all the equipment you need. Make sure you take a light source, as there is not much bio-luminescent fungus around... The apothecary might have something for you, if you have the gold, and if you are short of antitoxin and healer’s kits."

They are confused at first if there is a beast known as a "thresher".
But with a little more prompting, they discuss what dangers they have run across. At first it is trivial ("clothing stains that won't wash out") or exaggerated dangers they've heard 2nd hand ("don't think he could really kill a horde of stone giants single-handed"). This goes on for a bit, until you ask them to stick to dangers they have directly faced, and not tall-tales from other scouts.

• “This one time, the stalactites came to life and started dropping down all over us. Killed Jon.”
• “Sometimes you reach a crevice, and if you don’t have the gear to cross it, all you can do is turn back.”
• “We were just walking along when they came out of the walls, creatures of living shadow. Our weapons couldn’t hurt them, so we ran. Kylie didn’t make it.”
• “Watch the mushrooms. They can save your life. Or just as easily kill you. But only the mushrooms – other fungus is deadly.”
• “Sometimes, seeing in the dark wasn’t a help at all.”
• “This close to the surface, you don’t tend to see the big worms, but the little ones is bad enough. Watch the tails.”
• “Seen lots of webs down there. Never any spiders. I wonder sometimes if it was just the drow messing with us.”

At this point, they start to bicker amongst themselves about which of them has faced the biggest danger.
(Unless you had something very specific to ask, this is probably as much as you are going to get.)

Hildigrim gladly absorbs all he can, encouraging the scouts to talk as much as they're willing to by prompting clarification and (honestly) acting very interested in everything they have to say. All the while, his mouth is taking in his usual impressive amount of food for his size.

"So," he says when he feels the well has run dry, "we'll need to use my dancing lights and Jorin's glowing sword." He turns to Spar. "How is your sight when there is little light?" After her response, he continues. "We might should pick up a few more bundles or rope."

"Antitoxin seems like a useful item," Quillary adds to the list. "And something we have not really encountered before."

The scouts mention the travel plans that were left behind by the last two scouting groups that were lost.
They also give you instruction on common markings that might have been left behind by the scouts to give clues for others to follow.

When Quillary mentions an interest in the apothecary, the scouts add that he was around before even the Zhents were running the place.
The apothecary isn't an evil person, but is profit driven and has skirted some of the Alliance laws in the past.

Assuming you are done with the scouts, you head to the supply room to pick up more rope like Hildigrim suggested.
You get a 100' and a 50' rope, and put them in the Bag of Holding.
(The 3 GP cost is taken out of the party communal funds.)

You head to the apothecary to browse over his selection.
He has a vial of acid (40 GP), a flask of alchemist's fire (75 GP), 3 antitoxin doses (75 GP), 3 healer’s kits (8 GP), an herbalism kit (8 GP), and a pair of regular potions of healing (75 GP).
He mentions, confidentially, that he has one potion of greater healing in the back storage area, that he is willing to sell for 500 gp.
(Those prices are 50% higher than what you'd find in Waterdeep. And that greater healing potion is being sold for the price of a superior healing potion.)

Quillary steers well clear of the alchemist's fire, but plucks up the antitoxin, all three of them. She debates over the healer's kits, and for the cost, takes them, too. Then she goes back and takes the herbalism kit, too.

(OOC > it appears antitoxin is to be taken prophylactically, that is, before being poisoned? It confers advantage on saving throws vs poison. I thought it would neutralize poison, but that does not appear to be the RAW. Can someone confirm? Also, yes, she has an herbalism kit already, but this one is interesting to her in that it will account for regional, and perhaps underground, differences. Also, herbalism kit description says it is needed to create antitoxin. What else would she need? She is proficient in Nature, Medicine, and Survival.)

She goes over to a corner and plops onto the ground, examining her kits.

"Did any of you know entire scouting teams that went missing?" Spar speaks up before they leave the hall. "Or been on ranges where some people went missing, instead of outright killed? Are scouting teams going out now? If not, why not?"

(Then...)

"Where were these parties when people went missing?"

[Am I mistaken that these scouts are supposed to know more about the two teams that went missing, the third team that tried to find them and failed, and the fourth team that went to find them and didn't come back?)

She answers Hildigrim that she can see almost better in the dark than in the light. She also mentions, fleetingly, that heights don't bother her, and she'd only need rope to get across a chasm.

When the party goes to the apothecary, she reaches for the writ Ardragon gave to the group (assume Hildigrim has it, but she snatches it away) and holds it up so he can see, but she doesn't let go of it. She waggles a finger at him and waits for real prices. She doesn't let Quillary waste her money on overcharging, though she'll pocket the savings if Quillary doesn't want it.