I dont think reverse polarity would be the issue, im not really familiar with the story of yours failing.
Vibration or excessive PCB flexing tend to be a big cause of failures in things like car amps.
This is quite strange actually, the only reason the resistors would blow is if the gate of the FET went short circuit, which is rare is the gate input of a FET is capacitive.
Why would you go to Nissan for them? They are all made by SHL anyway, just go to a bolts + fasteners shop and pay $1.25
You should replace every stud and nut, reuse the thick washers.
Could someone please measure the length of one of the braided lines, I need to purchase a clutch line for converting my A31, I beleive R32 will fit fine but just double checking.