Re the plastic bumper, I think there was another guy who had some adhesion problems on his plastic bumper, but IIRC, this was because he did nothing to prepare the surface prior to painting (i.e., no sanding).

I can see how a smooth surface like that, especially one that shrinks and expands depending on temperatures, might cause problems if not prepped. So I would be sure to rough up the surface with some fine grit paper (maybe 400-600) to give the paint something to adhere to.

Re blocks, I didn't use any, but now wish I had. I had to do a ton of wet sanding and my fingers were literally bleeding at one point. But I have not of anyone using an orbital sander for sanding, most people tend to think it would be too powerful, but i have not tried using one myself.

Also, I burned through the paint in a lot of spots and suspect a block might help prevent that, at least on flat areas. Be sure to watch out for edges when you're sanding, it's really easy to over sand, so I would err on the side of sanding places like that too little rather than risking burning through.

Finally, watch out for drips. I let far too many of them slide on early coats, thinking I could sand them down, but it's almost impossible to sand them perfectly flat. Fortunately, since I used gloss white (probably the most forgiving color) they're not too noticeable as long as the paint has a good shine. The reflections really do a good job of keeping the eye from seeing them.

I think a week to 10 days should be enough down time to get all your coats down and then reassemble the car. The final wet sanding and all that can be done days or weeks after you finish laying down you last coat, and the car can be driven while it's drying. Good luck and post some pics when you're done.