First off, you need to read the tech sheets of what you are shooting. Second, there is a tech line at PPG you can call if you don't understand what is on those tech sheets.
1. Depending on where it is at and how well you want it to look you could do anything from just "scuff it" with a gray scuff pad to sanding it flat with 320. You are basically "painting", that is how you have to look at shooting a "sealer". You are "Painting" the car, how well do you want that area to look when done? The fact that you are sealing it you can step down to 320, other than you would want to be a 500 or so for paint, there is no difference, you are "painting" when you start spraying a "sealer".
If you want it real nice, take the time to use that green epoxy as a "filler" and sand it real nice and smooth, removing any imperfections you can.
That is of course if we are on the same page and you are talking about a "wet on wet" sealer where you will spray the sealer and then in the same session spray the paint.
2. Either way if fine, it makes no difference. But on the color issue, I would recommend using the one closest to the color you will be spraying, or at least the lighter of the two.
3. Don't know, I would say no. Most primers don't "tint" well, they may change a little but not a whole lot. There are "tintable" primers and sealers that do wonders but they start out very "opaque" unlike regular primers.
Again, call the PPG tech line.
4. READ THE TECH SHEET! This is a VERY important issue that is VERY well explained in the tech sheet. YES you will want to let it thoroughly flash before applying clear. And to wait until the high end of what they recommend the better.