Wordsmith,
1.) I believe the white stuff is called "chalking", and the fact that you have some, I believe is one of the indicators that the roof does indeed need cleaning. Get out your paper work on Montana and see if the cleaning is covered in the manual. We use Dawn dishwashing cleaner, you can also do a search here at MOC and find a lot of discussions on roof cleaning.
2.) spring?? no idea, I do NOT go up there, Al does. sorry, one of the others will come along and tell ya.
3.)Rust, sand, and paint, or replace. We had ONE vent that rusted, but we were still under warranty so it was replaced by Keystone.
I applaud your ability to even get up on that roof, personally I have never been able to get myself past the 3rd rung of ANY ladder!
Carol