It's just a bit of a retcon like everything else - in The Host the symbiont took over the host's body, in DS9 it's a merging of the two.
No need to over-think it too much, just the evolution of a concept when it jumped from a one-shot TNG episode to being used regularly in DS9.
Exactly. I think sometimes we all forget that these are just TV shows.
They should have just called the spieces on DS9 something completely new instead calling it Trill. It wouldn't have been hard to invent a new species similar to the Trill their symbiotes. Way to many recons happened by trying to make it fit.
I agree. Once they decided the forehead makeup wasn't what they wanted and the no-transporting thing was impractical, they should have just renamed it.