Inge's Polearm Master opportunity attack using her reaction when mephit 2 enters her 10' reach:

Glaive to hit:  23 with advantage (or straight, first roll of 17 if Brix's Faerie Fire is already gone)
Glaive damage:  10 slashing

(The above attack of opportunity occurs with mephit 2 still 10' away from the door, I think, in case it explodes!)

If mephit 2 survives, she still manages to parry its claw attack away.

Milo succumbs to sleep. It's much more peaceful than fighting a dust creature so why not. 

Seeing the second of the three mephits explode at a safe distance, Inge charges into the room with a roar after the third one who had just thrown more sand at her companions to make them take a dirt nap.  It is meant to be a fearsome battle cry, though it sounds suspiciously like "STUPID MEPHITS!"

Advancing up to her 35' move speed, but skidding to a halt 10' short of the third mephit, Inge whips first the blade of her glaive, then the haft around at it.

Movement:  up to 35' into the room, stopping 10' away from third mephit.  (Any remaining movement to retreat after attacking, back towards the door).

Action:  Attack third mephit using 10' reach, possibly with advantage from Brix's Faerie Fire:
Glaive to hit:  19 with advantage or 18 if straight roll.
Glaive damage:  8 slashing

Bonus Action:  Attack using Polearm Master bonus attack with back end of glaive, possibly with advantage from Brix's Faerie Fire:
Glaive haft to hit:  17 with or without advantage
Glaive haft damage:  8 bludgeoning

7

4

Brix slumps down and slowly falls over on his side. Pulling his tiny knees up to his chest he curls into a peanut shape. A loud snore erupts from his nostrils just before he starts to suck his stubby thumb.

Inge attacks the third mephit ferociously. The blade of her glaive bisects the mephit and she quickly spins the haft to slam it to the mephit's head. The mephit slowly reforms, but once again the dust in the room is noticeably reduced and less excited. The colourful dust on the mephit falls away as it reforms. It looks severely hurt.

{{As Brix is unconscious, the Faerie Fire drops and we move to the top of the round. Go ahead, Wadi. Milo is also unconscious. Myrla, standby.}}

Wadi will wake up Brix 

Inge is truly a force to be reckoned with realises Myrla, after she had dispatched nearly all of the mephits on her own. Sure, she needed Brix to see them but after that she seemed unstoppable. Hoping to finish the threat for now, Myrla steps into the chamber - trying to get a clear shot without Inge in the way - and nocks an arrow to aim at the last remaining mephit. Not wanting to take any chances, she puts all her focus in the one shot. 

Action: Longbow attack on Mephit with Precise Strike, hit 8 without advantage, 14 with advantage (granted by Precise Strike) for 19 damage

After stepping into the room, Myrla aims and shoots her arrow a second time. This time, the arrow flies true and pierces through the mephit. The mephit shudders for a moment before it too explodes into a cloud of dust. If anyone were close to it, Myrla knows that they would be blinded for some time. With the last mephit dead, the swirling dust around the room recedes and becomes still. The door frame where the others still stand flashes blue and whatever mystical barrier on it disappears. Some of the still swirling dust starts to spill out of the room.

Inge spins her glaive and stows it in beaming satisfaction, her expression like that of a child who has just made a good play in a sporting contest.

She trots back to the door, wakes up Milo, and begins pulling the cart through, not really considering whether there might be other perils in the chamber besides the mephits.  "Come on, let's go through before the dust all spills out and makes every place in here look like the White Waste."

Her words are a clumsy cover for ulterior motives.  Anyone watching Inge can see her hand begin to sneak towards another of the rations on the cart.

Milo stirs, "What is it, can't I have 5 more minutes..." before realisation kicks in that he was in combat and it wasn't sleeping for the night, and he leaps to his feat grabbing his swords "Where are those mephits! I will get them for that!"

Brix scrunches up his face and and smacks his lips. Removing the thumb from his mouth he looks at it with confusion. He rolls about a bit before finally working himself into a standing position. Brushing himself of he comments, "What next?"

"Let's see what's in the room we just cleared," replies Myrla brief and efficient to Brix who really seemed to have been in another world for a moment. Occasionally, she had wondered if it is actually nice to sleep. So many races do it and they seem to enjoy it, otherwise why would so many of them have such a hard time to come back quickly? 

She steps inside the chamber and immediately turns south and tries to figure out if there is a connection to the other door, the one that was melted into its own frame. On her way to peek around the corner, she inspects the room with a cursory glance (Perception: 23)

Looking toward the room beyond the archway, Myrla sees a dimly lit room. To the west (looking right from the archway) is a corridor, most probably leading to the room with the melted door. As she squints her eyes to see what is in the dimly lit room, Myrla sees a coffin of a kind hovering about an inch in the air. She slowly backs away when she notices something beyond the coffin-like object, glinting white scales as large as her shield. A quiet growling breathing echoes faintly from the room. 

I suppose we must push on if any answers are to be found. says Brix as he follows Myrla.

“Something big with large white scales is further back in here!” Myrla advises the others. “Let’s be cautious.”

"I wonder if that is what melted part of the other door."  Inge's whisper is almost reverent after hearing Myrla's description.  In a rare turn of events, she forgets her (not so) surreptitious gambit to swipe another ration from the cart, and her childlike eyes grow wide.  She remembers listening rapt to her village wise woman tell of myths, perhaps from different worlds and eons, in which giants battled with dragons for dominion.

"Did you see anything else besides the big white scaled monster, Myrla?"

{{Myrla, with that 23 Perception, I will say that you also notice the creature to be strangely stationary. You've worked with living beings, humanoids or animals, for a long time and you know that no creature would remain that still. Even predators waiting for prey or sleeping would not be that still.}}

{{Also a reminder that technically you still have another door if you don't want to go forward. Melted to the frame it may be, but I did say that a good strong push would open it.}}

Inge shifts her balance from one foot to the other.  Her shoulder itches and she scratches it furiously.

"You want me to go in and check it out, Myrla?  Anyone?  Or back to the other melted door to push through?"