OK, is the disintegration in the engulf negative energy damage? I think if we figure out what the damage is, that write-up shouldn't be too hard. I guess the only other question is if there should be any way out short of killing the quasielemental.
Typeless would make sense, but these are negative-energy-linked critters. I guess I'll go along with typeless if you prefer.
I guess Engulf often makes it pretty hard to escape (the gelatinous cube isn't too specific), so I could go along with making it impossible to escape short of killing the quasielemental. But then we need to limit the size of engulfed critters. Maybe to 1 size category smaller?
I prefer the save, too. But I think you've missed out on the any single opponent, right? It reads like it can keep engulfing new opponents. Also, I like that the victim can escape via the grapple rules, but I thought you were against that.
I'll agree to all that. I'm wondering, should we specify if the "end of each round they remain engulfed" is at the end of their turn or the end of the dust quasi-elemental's turn? Also, I'm guessing at Con-based saves.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.