I know you mentioned its at the repair shop now and out of your hands.
But I'm curious, did you check the converter reverse polarity fuses? You mentioned intentionally connecting the battery backwards as part of your trouble shooting, that should have blown those fuses.
I think you are right and there is a wire fault on the 12v side, but regardless the polarity fuses need to be confirmed good.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)