Spam is usually dealt with at the MTA level. And, honestly, I usually don't deal with that sort of thing (my personal feeling is that one mistakenly flagged message isn't worth any number of correctly flagged ones). However, I cannot imagine anything that procmail could do that Sieve couldn't (but I'm not an expert at either one, so I could be wrong). Check out more information at http://www.cyrusoft.com/sieve/

You can certainly install it alongside another IMAP server, and since it won't be using the same store, you won't even have any concurrency issues. However, you'll have a minor problem in getting mail to it. It's easily solvable, but you'll need to figure out the best method for your testing (certain addresses delivered there, separate SMTP server for it, etc.)

I think the best way to go about archiving is to set up a script that moves all the mail around using IMAP commands, like I suggested above. In fact, if you're looking to do this for multiple users, you could set up permissions on everyone's mailboxes so that a particular administrative pseudo-user has access to do this for everyone. Automating it should be trivial.