Ember: So, everything went fine.
Thorax: Everything went fine? He held me above a volcano!
Ember: Well he didn’t throw you IN the volcano so I’d say he likes you.
I’m betting that changling shapeshifting is limited to gaining a creature’s form, but not necessarily it’s attributes or abilities. Otherwise they could all have turned into things like Dragons, Alicorns, and Dracoequi and the heroes would have stood no chance against them.
Even if he could, he would still drown.
Fuck diplomacy.
No idea if the disguises are that effective.