What a pain in the backside for you! Surely a 'certified' copy is every bit as good as the original. (That's why we pay for them.) It shows that someone who is able to have that responsibility has certified they've had sight of the original; that's all that's needed. I had deputyship for my mum and, in the early months when I visited banks to transfer the accounts over to me, they viewed the original (of which I had several), with the court seal, and returned it after they'd copied it and certified it themselves with a stamp as being 'seen'. If you only have one copy of an important original document of course you want to keep it in your own hands. That's what certified copies are for.
It shouldn't be your problem that they can't see the 'original stamp' on their own photocopy. They've had sight of the certified copy - all they need to do is stamp on their copy that this was done. I hope you get to deal with somebody who actually has a clue. Fingers crossed.