Is it fair to assume the OP should have had prior knowledge of PGX's reputation? I belong to a couple of Facebook comics groups and there aren't enough hours in the day to correct the ignorance of PGX's "standards" there. Isn't it perfectly reasonable for him to assume he is getting a complete book unless otherwise stated?
As far as chain of custody goes, that's a tricky one, kind of like discovering you were given a counterfeit bill in your change. Although imperfect, I think responsibility can be shifted to the source; I buy it from a dealer, I shift it to them. If they have records they can pursue the person they acquired it from, all the way back to whoever subbed it in the first place. The lesson here is keep good records for significant purchases.
As far as the cracked slab goes, I view that as a wrapper the book came in; many people crack upon receipt and that is not unreasonable, in fact it's the only way to discover the book was damaged. I'd say the OP is owed his refund, and if the seller is hell-bent on having it slabbed by PGX he could deduct those costs from the refund. Kind of an uncool move but I'd allow that if I had been the OP.
My apologies if these sentiments have already been expressed, I have read most but not all of this thread.