You could backup the data folder(in your controller installed directory) then cover the new installed controller. Or you could also backup configuration file in System-->Backup&Restore.

First, check whether your firmware of 901_v3 is latest. http://www.tp-link.com/en/download/TL-WA901ND_V3.html#Firmware. Then make sure that 901_v3 is the same wireless setting as the former 901_v3....

I would advise that you could use External Radius Server or External Portal Server when you want to use your own portal. The Hotspot authentication cannot redirect to pages you want.

In the future it would support Linux. I just asked before and get answer from TP-Link support. Maybe controller 3.x.x

What do you mean the PLC, is it the TP-Link product? If so, keep using PLC.

You would better provide your network topology first. And you need to check whether your controller firmware could work with EAP firmware. See in the official website. Then you need to check whether...