Although the primer is not as substantial as one you'd apply, I don't prime them either (unless I'm covering a prior color). I use Rustoleum paint usually on those doors (ours and rental). It holds up very well, especially on rentals which take higher than average wear ;) Our garage service door is white Rustoleum, and has held up very well. Recoated once in 12 years. Be careful sanding, runs the risk of pulling the existing primer on there off.

Black siding huh? I think it's going to be hard to maintain whether you use oil based or latex. Black is just a rough color. Oil based products fade faster outdoors, they are more brittle than latex and mold/mildew grow easier on oil based paint, but I've never really seen a black latex paint hold up well outside. I would lean towards the oil and would probably choose Sikkens, but I would also consider choosing a lighter color too. Oh and just because they use oil based paint doesn't mean they can paint when it's 10 degrees. I hope they are heating that garage during application.

Centsational Girl had a good tutorial on how to paint furniture just the other day... Here is a link that might be useful: How to paint furniture

Seems to me that painting over the white with black paint would be the easiest way to deal with it. I wouldn't want to try removing the white paint as you could end up somehow compromising the shingles' effectiveness (scraping, etc.) and why work that hard if you can just go over it?