Man I don't know where to start, we clearly haven't had the same experience of BG3.
I don't understand what you find childish about the absolute. I actually like that the fact that it is downplayed during act 1. It acts as a nebulous threat during the playthough and it connects everything together. I am sure later acts will quench your lust for an explicit BBEG.
All your companions have a reason to act the way they do. Laezel is even very adamant to find a cure contrary to what you said. Astarion is enjoying its first moments of freedom in hundreds of years and is happy to use the tadpole for his benefit. Shadowheart is clearly brainwashed to do her mission no matter what. Wyll has to keep his facade of cool. Gale was already a ticking timebomb and is also looking for a healer.
I don' t understand what you say by the tadpole being "gimmick, having no dramatic purpose or interesting undertone" when it is the main motivation and source of power of your cast.
I'll give you that Raphael is a bit flamboyant, but hey, I think it serves the scenes well to imply its powers to the player. Gotta impress your public if you offer a deal for someone soul.
For the funniness of the companions, I had some genuine laughs here and there. More would make them fall in your "corny" basket.
