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?
Drow can go outside. It just their gear that get dusted. Its takes a long time for them to accomodate to sunlight, though. Its hurts their eyes.
If it's what it's takes to save the world, then the world doesn't deserves to be saved - Geralt