That plastic cover is pretty unnecessary, mine's busted into 2 pieces with more holes in both, and I know many that have just removed it. Don't worry about it.
Don't change a thing about your wheels, those 285-width (nearly 11.5") tires can only just tuck into the rear wheel wells. Switch to anything that will move them further out, and you'll get rubbing here:
That was just from the factory +40mm offset 15x7 wheels to older Nissan 15x7s with 25mm offset (.6" further out) with the same 32x11.50-15 tires. Yours are as big if not bigger (15" or 16" wheel?) and are going to be as tight or even tighter fit.
If anything, you could install bumpstop extensions for the rear to limit the uptravel a bit.
Brent