I have seen posts where others have relaxed the truss rod, then clamped the neck on blocks to counter the back or forward bow.
I've done this to some extent, with my EXA-1 and my SV-1. both of these had a back bow and clamping the two ends and leaning my weight on it was enough to straighten it out. I added a third clamp to the middle to hold it like this overnight and then just nipped the adjuster nut up.

What is concerning is the lack of apparent adjustment you've noted.

If it is a dual action rod, you'll not get the nut off anyway as it is threaded l/hand one end, r/hand the other through two square nuts, to which a flat piece of 2mm steel strip is then welded. The twist l or r either pulls the nuts together, bowing the flat rod and creating back bow or drives the nuts apart, bowing the threaded rod, thus providing forward bow.

The ones that I have seen in the PBG kits (Because i am the kind of tinkerer that will pull things apart) are functional, and reasonably well made. Certainly there wasn't a great deal of difference between them and one i have bought for sctrach builds.
It is possible a single action one has snuck through.
The welds on the adjuster nut could potentially have caused some weld spatter to fouls the threads, but you'd have to have been horribly unlucky if that was the case.

Its possible that some glue or something similar has fouled the adjuster thread, but again you'd have to have been horribly unlucky.
Hopefully that is informative, if not entirely helpful for you.

As cringe inducing as it is to go back to Adam again, it may be a case of having to do just that. If it can't straightened with the truss rod as is, then it would have to be considered faulty.