Congratulations!!!
You have entered the world of wonderful ZR-1's!!
I would not be concerned with the rattle at this point.
The reverse lights even less pf a concern.
New switch will fix it.
The plenum is going to have to come off in order to determine what someone has screwed up.
Should not be a big deal.
Plenty of " How To" info available for you.
You purchased at an admittedly good price.
Now invest some of what you saved to correct what sounds like a hurried job done by an amateur.
Keep us posted on your progress, and let us know your questions as you proceed with the repair.
WELCOME!
Marty