I THINK the time passage issue could be solved in a couple of different ways, but I have no confidence in my ability to estimate how viable this is.
One way (perhaps a crude one) would be to measure time passage by "not in TB party member(s) time flow". If it makes sense. Or the opposite, stop/slow time passage (in terms of time of the day) to match characters currently in TB. TB, FTB and mixed-mode party make the game world function in a weird temporal shift anyway, so I don't think it would make it worse. This would be sooort of similar to your suggested real world time passage, in that it would unify all party members' time.
I do realise that fully implementing day/night cycle and weather would be a massive amount of work, but imo it would do wonders for the game. So much more reactivity. Rain douses flames. Wind dispels clouds quicker. And let's not forget: Larian have underlined the significance of light/shadow in the game, and those systems would add so much depth to it.
To add to that, it would give a strategic layer to rests - perhaps limiting long rests to 1/day on higher/"survival" difficulties.
Great points I'd also add that, if you've chosen to play as a Astarion or a Drow, how are we gonna walk around in the sunlight when the tadpole is removed? Or is it never going to be removed at all?
Probably not... it's kinda like being a Baal spawn. A gimmick to drive the story and explain special, homebrew powers. The final decision our character will probably make will be to either a) remove the tadpole or b) become an illithid elder brain, or something, but no matter the outcome, the story will most likely be over right after that.