It varies from site to site how difficult it is to create another account. Most sites don't allow you to create another account for rakeback if you are already signed up with them. There are ways around this usually. Do a search of each site you are interested in and I'm sure you'll find a thread that is helpful. It is not a good idea to create multiple accounts as this is almost always against any major sites terms and conditions and could lead to you having an account frozen.

If you are only hitting bonuses you won't earn much, if any, rakeback at most sites since your MGR will be negative or very small. Rakeback becomes more beneficial the more you play after hitting the bonuses. Rakeback is a bonus and bonuses are rakeback. In either case you get a portion of your rakeback.

Like RTR mentioned it is always a good idea to sign up for rakeback when opening a new account. It can't hurt you.