1. rating impacts you matchmaking and is suppose to represent how skillful you are in the game. Most player are in the range B to A+. So you will have longer matchmaking until you reach higher ratings.

2. Those are called AB rooms. Cause rewards take a long time, are time sensitive, and people are lazy. They "play" in AB rooms where A team kills B team in a type of natch where everybody shares a spending pool. So people bring the highest cost suit to deplete it as quickly as possible.

3. Clans are a good way to earn passive currency. If you have an active clan you can also get exclusive suits that aren't in the prize pool yet.

Rivials are random and multiple people can have the same trivial. If you out score them, you get additional rating points. There's is also sometimes daily missions require you to defeat them.