The purple 10 minute suggestion in Devour is fine.
DR 5/piercing or slashing seems good enough, along with resistance 10 to fire.
I think I'd like to leave the save off the Sensitivity to Cold. If the PCs figure it out, let them use it!
Updating the Ihagnim.
Could we just put the Incomprehensible bit in the flavor text?
Sure, there's precedent for that.
Did you have suggested text for the gravity vulnerability?
Have to think about this.
Would also like to make it dependent on the Astral Plane somehow.