So yeah, your solution is probably more stable. But if Larian has coded the game to make everything already depend mostly on the host player (idk hpw they've done things), then it might be easier to control a day-night cycle based on their actions.
As you say, these edge cases aren't a big deal and would be outweighed by the benefits of a day-night cycle.
Oh, to be clear, I would STILL count the host as main reference. It's just that everytime someone enters a turn-based combat his "background clock" would stop ticking.
Frankly it's not even something the player needs to be acutely aware of, just a minor hidden tweaking in the grand scheme of things. Let's say that you'd set for a full 24H cycle in game being the equivalent of, dunno, 3-4 hours?
In this example as the host roams around it's 11:00am, he walks some more and it's 11:15am, etc... but then someone enters combat and while he stays in real time, his "background clock" will freeze at 11:23 and stay there until the combat is resolved (be that with or without his direct help).
The actual swap between night and day is a different matter. That at Larian's discretion and according to what the engine actually permits, could either be a dynamic and gradual change of lighting OR a completely binary swap to a "night version" of the map, with a short dusk/down videoclip hiding the loading. Which is pretty much what BG1 and BG2 did back then.
