The OP was trying to "Bush" some of the holes, those that were not broken out to 2".
Also, just a question, if you can get to the back side for a nut and a washer, why couldn't it be a bolt or a machine screw?
Also, the trim backer would have to be attached before the foam/ vinyl which to me makes no sense. Doing it the way I pointed out, the boards could be fitted up, removed, foam/ vinyl installed using Monel or SS staples, then reinstalled as a finished trim piece with no fasteners showing at all...
none, plus, if you ever need to remove panels for any reason, one could simple remove the bolts from the back of the panel, and reinstall when finished.
I'm not really sure, how your foam/ vinyl attaches. Looks like there is no way to remove yours without trashing the vinyl. Also, why do you need an additional nut when that is the function of the "T" nut? The backer panel is sandwiched between the "T" nut and the fiberglass panel, so they won't go anywhere and the longer barrel will help bush the hole as the OP wanted.
Even with the back-up washers, they should work as drawn.