I think a mention is necessary far earlier than whenever Omeluum comes into the story (I haven't reached that point yet and have thus far played all the way to the part where we enter the Underdark). Mention of it should come soon after seeing the tadpole leave the dead True Soul. This question, to my mind, isn't meant to be a mystery for the player to unravel. Therefor it should be made clearer for the players so that they can be on the same page with the game. It's really as simple as someone saying "with how strange the tadpole has been thus far, we shouldn't rely on assuming such a risky tactic is going to work out for us." Boom, just like that the audience understands the parameters of the narrative and aren't left wondering if the game is ignoring the obvious answer or not.
As for the issue of not everyone in the party having much to say about it, I think it's just a challenge for the writing team to deal with.
