I’d say if it’s a road open to traffic then you should never cross the center line. It’s just dangerous and not worth getting pancaked. Especially since you said there are big trucks with trailers and construction vehicles. Not to mention that it’s probably illegal to do that.
Per the fighting for wheels. I don’t think I’ve ever had that on a group ride. I probably wouldn’t be that cool with it if it was anything more than just a bit of crowding.
And I agree with what you’ve said about the hammering on slight declines. It is always super strange to me that it is 100% okay for people to smash up a climb and drop everyone but it’s not okay to do that on descents. If you opened up a gap on a 1% decline then unless you were full on sprinting it was the fault of those behind you. At those speeds you are easily doing 100W more than them if they are in the draft. So they either were coasting or just weren’t paying attention.