you can add more than 1 adsense code but maximum 3. And yes u can add it in a forum as long as you own it with your own domain name.

It depends. For me I created my adsense account using another google acc so it is different. But if you used the same account to register for both blogger and adsense then it should be the same.

Even if it isnt the same you still can use you other account to include ads in your blog.