This class makes me wish I could find a 5E game to get into (and the time to play).
I do have a couple of questions, though.
First of all: what AC does a swallowed creature attack against? RAW would seem to suggest the same AC for people attacking from outside, but how would your scale mail (for example) protect someone's insides? Maybe it could use Unarmored Defense, with the excuse of the ozodrin's stomach being just that eldritch and protean? Also, in the interests of helping Swallow remain relevant at higher levels: does Stomach Realm permit the ozodrin to form natural weapons internally to deal extra damage? This could possibly be another Advanced Alteration, so those dedicated to using Swallow as a combat strategy would be investing more into boosting its effectiveness.
Also: Many of the base alterations permit the creation of additional body parts but no limit is mentioned. This probably matters most for puppet, which (rules as written) permits infinite Toy puppets capable of carrying 1 pound each. Presumably the DM'd veto carrying off the entire dungeon this way, but where's the cutoff point?