I think you can go up to 3.5 deep. Not sure tho. I use a 3/4 spacer with my focal k2p.
I only run front speakers so I can't speak for the rear, but I made my 6x9 adapter plate out of half inch ABS and I can't imagine any speaker being too deep for it after that. I have JBL Gti660 is my front doors and they still have plenty of room to spare
Should be fine. I did use my own spacer I cut it out of 3/4 mdf. I guess it's a good idea to paint it too. I haven't had an issue since I used dynamat over it.