I think the simplest solution is what you mentioned: if anyone in FTB -> stop clock (I actually suggested this some time ago) and all players must agree to rest. I may be missing something, but I don't even think with this we'd need "host clock"? All time passage would be solved either automatically (FTB clock-stop) or by player agreement (rests).
I'm also very much for "wait" option. (Perhaps could be treated as short rest? And long rest could have an option to increase the time, so anywhere from 8 to 23 hours.) This would allow for more strategic use of time of day (should Larian decide to do this and spare some extra work beyond cosmetic day/night).
When it comes to time scale: I think a factor of 15 would be fine. It's the one The Witcher (1) uses and I found the time of day to not change too quickly. It's 96 minutes per the full day cycle (similar to TW3 with 90); Skyrim is 72 from what I found. Though one full cycle in BG1/2 was 2 real hours and it could be good for BG3 as well (since you skip time often anyway by resting, so there's less need for quick day cycle for variety).
