Hello and welcome to the forum.
With all these people including docs talking celiac disease, have they ever done the blood tests on your youngest daughter?
As for the seizure being the 'trigger' for celiac, seizures can be part of the celiac picture as an expression of celiac - a neurological manifestation (this kind of presentation is not at all unusual). Blood testing is pretty simple although not always reliable in infants, but worth a shot. I must say I have never heard of puberty in an infant and don't know what to make of that.