I don't know about the other questions, but I've been told by dedicated AC shops that the caps do help as seals, they also help keep crud out of the schrader valve in the fill ports.
Also, you might see if there is an AC shop in your area, they often have some generic AC caps that you can pick up off of them for a song. I think I paid $2 each. Admittedly, though, that's small change compared to the rest of the work.
