If you have multiple links from a site or forum, on different pages help in the serps? Assumin it the site or forum is related.
Yes, only if you have more links on the same page it will be counted only ones. I remember reading this somewhere, but I can't remember where.
I don't think it's true. For example try typing link:www.yahoo.com in Google search box and in the results you will see www. flickr .com appearing 3 times on the first page, 4 times on the second page etc. I doubt that Google would show links from the same domain if they where counted only once.
Yes, but really no. In theory, a unique backlink is from a different domain name; having multiple links from the same domain name would just be considered duplicate backlinks. However, consider content can vary web page to web page there is some SEO value behind it. Remember, quality over quanitity.