A New World

So, would Ch'aska and Landon be able to understand modern draconic?

Good question: I'd say Landon doesn't know modern draconic, though as a descendent of old draconic it might have some similarities he will notice, or be able to deduce with a flat knowledge roll. If he doesn't actively try to understand them, he won't though. If he wants to regularly understand different languages, he'll need to take a utiltiy talent for that (such as Many Tongues, I think?) or at least get something like a freaky knowledge in "linguistics" or such.

Oddly enough, the "common" we're using in this campaign might be even harder for Landon to understand- as it's even more distantly related to Old Draconic.

As for Ch'aska, it would be unlikely that she knows draconic- the llamas keep to themselves for the most part. Still, its not impossible. If you think its important to your character's backstory, I might let you: Or at least I might let you make the same Knowledge roll (or maybe a Streetwise roll, in your case) to follow their conversation.

A reminder: The hall's size is not conducive to flying- at least not of a dragon of any of the party members' sizes. You could attempt to fly across the room if you think your skills are good enough- although naturally failure would be very bad. Also, you'd at best be able to fly straight across the room, into the doorway with more stairs leading down: there's not enough room to turn to one of the side doors.

Building on what you're saying about languages-
If Landon can't speak common, and Ch'aska doesn't know old Draconic (which is distinct from common)
Then they shouldn't be able to understand each other at all. Or am I wrong?

Incidentally, Curunir, could I use Rufus to scout out the stairwells? Moles are really remarkably small, with the adult star-nosed mole, of which Rufus is, is about 5.9-7.9 inches long, and, at a glance, about 4 inches wide, if that. I'm pretty sure he could skirt the edges of the lava, given just how many inches there are at the border.

Hmm, I suppose that is an issue. You are correct, the way I've been handling language. For convenience's sake, let's assume either Piasa or Arcantos translate between Landon and Ch'aska, unless anyone is specifically leaving one of you out of the loop for whatever reason. I suppose that would become an issue if just Ch'aska and Landon are left to themselves.

Rufus could definitely fit in the space between the walls and the lava.

Arcana check,
13+12+1(g)=26

Knowledge: 10 + 8 = 18

The skull's voice in your head sounds a little weaker and distorted, as though something were interfering with his ability to communicate with you.

"I wouldn't be surprised if the priests were immune to the lava's heat, though I think it more likely there's a way to temporarily drain the pool.  

I will need to go dormant soon- this is the most talking I've done in a while, the mad griffon never listened to me:  I need to regain my strength.  It's probably for the best, since I'm sensing some odd necromantic magic somewhere up ahead, already draining my energy.  If I'm still awake it may seek to keep me out, or even control me."