That's my suspicion, too, since there's some "recover app receipts" functionality I've found references to here and there. This prompted me to wonder what exactly that means, in terms of - do you get an identical receipt to what was there, or do you get a newly generated one with possibly minute differences that would yield a different hash

It's a long story. Essentially, I want a unique key on the back-end before I make any calls to Apple Server APIs, but there's also legacy to it. That's the thing I am trying to prove - that using the hash is not appropriate due to regeneration. Txn ID is in the AppReceipt container, yes. There's a slight oddity with getting the ID via official libraries though, in that it only extracts the first TxnID, when the InApp collection is an array, and thus could contain Txn IDs for different products.