Why doesn't time end first frame you enter the elevator? It makes no sense to end time when the screen is fading. Load times between Act 1 and Act 2 can also be differing depending on how fast the other person loads in so that should be taken into account into. You can also start moving around in the small room during the long cutscene at the beginning so technically it's really hard to know for sure whether you start time on first actual input. Just to avoid any issues there I advise just starting time as soon as chat shows up because it's impossible to open any doors or interact with anything during the cutscene anyways.

So I'll clarify a bit the retime rules

For All Acts time starts when the dot in the middle appears and ends when the elevator ends. I have asked Oreo about this and he said that end time is like that since that's when the game actually ends (from what I understood)

For Act 1 its when the dot appears in the middle and ends when you get teleported (aka when your screen goes completely black).

For Act 2 its when the dot in the middle appears and ends when the screen starts to fade after entering elevator.

Now for load times between Act 1 and 2 I haven't asked why we aren't taken that into account, but it could be due to it being a hassle to deduct retime time with that (though I'm not sure since we didn't talk about this).

Also sorry for the late reply!