The holes on frames and whatnot can be off a bit sometimes, it’s not uncommon. One of the reasons why aftermarket manufacturers design into their products slotted holes is to compensate for that. Some manufacturers don’t which can lead to problems like you are experiencing.
If you are having problems lining up a hole or two, and all other bolts are really lose then you may just have to grind out a hole to get it to fit. Also look for some slag or sloppy welding that might be keeping the bumper from lining up too.
On the plastic cover you ask about I’m assuming it’s the top piece. Usually it is not reinstalled because it will no longer fit.