^^ I agree, but that doesn't mean we can't ignore that and discuss things totally "in-universe" for fun. (Although, I feel like there are a couple of "cues" in UBC's appearances; he seems just a touch colder. But that's probably just reading into stuff too much.)
I like your second idea, about mature changelings, datalogan
. It explains how Martok-changeling was able to avoid detection for so long. It also add an element of hubris to the Federation's assumption that Odo is indicative of all changelings.
And I suspect that the "baby" changeling getting reabsorbed may very well be part of the changeling "life cycle." (The novels sorta kinda suggest this. Sorta.) And so UBC might have felt that there was no need to intervene and just happened
to allow the "baby" (which may, in fact, have been very old; remember what Shakespeare said about "second childhood"?) to merge with Odo, reactivating his metamorphic abilities.