To add to what u/nh1402 said about #2, if it's anything like racing F1 cars in simulation, it's not as crazy as it sounds if you have a lot of practice. Right before the start you know who is immediately behind you and who is in front, the crucial moments are roughly the first 3 seconds after the lights go out. After that everyone sort of knows where they're going to slot in.

So lights go out, you know almost immediately if you're launching well or you're bogged down. If you got a good start you don't have to stare your mirrors down much, you're more looking forward at the cars ahead trying to get a good setup into the first turn. If you got a bad start you want to watch your mirrors more, but you don't want to move much side to side because people who are passing you from behind have to move quickly as not to hit you.

On a related note, when you get really really good, a lot of times while braking in a tight pack you're watching your mirrors almost equally as much as you're watching the road in front. It's muscle memory, you've braked so much in that particular place that while your body is automatically doing it, you're watching what's going on behind you. It's extremely effective at preventing you from being hit from behind by an out of control driver, which is incredibly common in online races.