[POS system, EMV/NFC questions]

What I suggest is stay away from NFC for your customers safety. Also trying to add those features are going to be a power drain. Easiest method would be to keep it simple and get a square reader or a quickbooks go reader if you use their accounting software.

[Can't get Kali Linux USB or SD to boot on Chromebook]

I know Kali is the flavor du jour but I highly recommend Arch Linux. You can run it on any tablet, computer, I have even seen it run on a phone. It is extremely versatile. I have it running on a raspberry pi right now as well. It is a great operating system that will run anything you need.