Re: very hard to verify a cashless system.
No it's not. The key element of verifying the transactions is that the cash matches the receipts not that the double count matches. Cashless transactions still have the receipts, and should still retain enough of the account number to uniquely identify the missing transactions.
Where cashless loses businesses money is when the card holder claims the charge is bogus and the vendor no longer has the record to prove otherwise, or it simply isn't worth the cost to challenge it.