Let's clear some things up here:
If your using the AIO hack it is impossible to mess with any of the other stuff. Impossible.
Could a dealer void your warranty, sure. If he wants to be a prick about it. But this person who was denied could have easily rolled his changes back.
As to the cable, once you've hacked version 502/504 with a cable, you don't need it again. The guys working on this have something that can preserve the hacks. So I could run that and have my software (not firmware btw) updated to 504 and my hack would be safe. And once you've hacked it the easiest thing to do is...not get anymore updates. I'm staying on my version now until there's a compelling reason to upgrade...like AA. I haven't seen any features added on any update yet.
"- AIO offers other stuff too - in case in future you need to upgrade/add: you'll need the cable."
No, you won't. Once you do the cable hack it enables the USB port. That's what it does. So you can rerun the AIO from USB as often as you want.