I chose to use epoxy. I love the stuff called Plastic Welder it works great. Available at WalMart.
Trim with a Dremmel. My #1 tool for a Fett project. As for sandpaper start with a 60 grit and work your way down to a 200 grit. Thats about as smooth as you will need your armor pieces.
Dremmel and hobby knife for the tight spots.
Yes. I would recommend a light sanding on any of the surfaces you will be painting.
Not really a consensus, but I offset mine.
Be sure to show us some progress pics.
Gator