You can use as many websites/forums you want. There is no problem. But you can't have multiple Adsense account. That's against Adsense TOS. Best of luck!
Actually you CAN have multiple adsense accounts (legitimately) but only one can be a personal account, the rest need to be business accounts.
Yes you can. Its to your advantage because it will save you time when putting Adsense on other sites that follow the rules. Have multiple sites for each website would be very hard.
Once you have approved your adsense account you can use it on any other website and even parked domains. Don't place them on websites which are against TOS of adsense.
As long as you have ad blocks on the website that you originally signed up with, you should be fine. Yugo,