Sir Vecken of the Broken Horn wants revenge against Viallis, but understands he isn't in good enough shape to help in the fight, and that the other 5 need protection. After grabbing weapons from the fallen cultists, he is willing to lead them back to the surface and head to town.

(He probably needs some instructions. If you don't say anything, he might dump Young Fargo at the closest farm house he can find.)

Quillary tells Vecken and the Mayor’s sister to take Young Fargo to Old Fargo. Then she spins her club, ready to go.

(DirtyDogP's account was last active 4 days ago, and hasn't posted since the end of June.
Hopefully just extended holiday stuff.
Mingofaust and Guiltysparc can make plans,  but we'll slow down until everyone is back.

Does the party continue south to investigate the rest of this level, or back up to the upper level?
If the latter, to the locked west door that Viallis uses, or the double doors for the temple/altar?)

Quillary is for invading Viallis' personal space. If he is there, we get him mostly alone. If he is not, we might find something useful in defeating him. Now that we know that is his space, she is inclined to be more wary about traps, magical or otherwise.

Sorry. I was out of town and fully expected to keep up with the campaigns while away, but it didn't work out.

Hildigrim taps his chin with a finger and then nods. "It seems prudent to immediately face Viallis. If we wait, we may wear ourselves out."

The group leaves the south passage unexplored, and heads back to the stairs. You make it to the entrance hall. Seeing that there is no activity, you head to the west door, while the 6 captives head north to the exit to the surface.

The 6 freed prisoners exit the tower, and head towards town, carrying the unconscious Young Fargo. With the advice of Quillary, they will be careful on the roads, to not get recaptured by any cultists roaming the Trade Way.
(One of them asks Sir Vecken why he was near Greenfast, when he was in Waterdeep yesterday. They are too far away for you to hear his response.)

The party is at the west door, which you are told leads to Lord Viallis' room. It is locked.
(Hildigrim can try a Thieves Tools (DEX) check vs DC 14, adding a d4 from Guidance.)

(While i am sad to leave part of a dungeon unexplores, facing Viallas is for sure what Jorin would want, and also escorting the prisoners back out, at least part way, was the right thing to do too)

Hildigrim once again works the lock with his tools.

DEX + proficiency + guidance: 12

Hildigrim is unable to pick the lock.

Jorin can try to force the doors open (Athletics (STR) check vs. DC 17, with a d4 from Guidance), but it will probably make noise.

(If someone else has a better plan, or if you prefer to move somewhere else instead of this locked door, then let me know.)

Quillary kneels down to peer at the lock. Is it a normal lock with a keyhole? Or is the door instead barred from the inside? If one were got inside the room somehow, would one still need a key to unlock the door? She asks these questions of Hildigrim, lending her Help to investigate this.

”Do you recall when I turned a body to mist? I can do that again, and one of us can seep into the room, but it will not be helpful if the door remains locked. Then we will just have locked someone in.” She muses, “Of course, that condition can last a long time, up to an hour… someone could at least scout the room, though the gaseous form is not naturally stealthy, nor immune from harm…”

"If they can seep in, then they can seep out after scouting, even if unable to open the door.  I think this is a good idea."

Quote from Mingofaust >>

Quillary kneels down to peer at the lock. Is it a normal lock with a keyhole? Or is the door instead barred from the inside? If one were got inside the room somehow, would one still need a key to unlock the door?

(The module suggests a key is needed.

But if Quillary is willing to use a 3rd-level spell (Gaseous Form), then I'll say that the person inside can "back-light" the lock for Hildigrim, allowing him to see it better. He can then open the door without needing another check roll.)

(Perception check from Quillary and Hildigrim, as they were futzing with the lock.
No Guidance and no advantage.)

Quillary's Perception around the lock: 25 

Perception: 26

Quillary notices it is quiet inside that room.
Doesn't rule out that someone might be lurking inside, but she doesn't hear any movement or activity.

(Still need to hear what the group wants to do next.)

“I think there’s nobody there. Break it open,” she says to other people who are stronger than her. 

"You need not tell me that twice!" Jorin says, and puts a heavy boot into the door.

Athletics: 27

(ka-boom!)

(OMG. Quillary even is impressed, and she's usually pretty "the universe works weird, so this is normal")