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.