> So would it be possible to install software or hardware which always returns a "success" from the card reader hardware without it actually doing the transaction and give everyone who uses that machine free food until someone figures it out? Hmm…

Almost certainly not on EMV certified card acquirers, unless there is a bug in the firmware. These things are so locked down that, even as an authorised developer for the payment terminal, we had no access to the hardware, no way to view the cards encrypted payload, etc.