Nope, WPA9610 doesn't support WPA / WPA2 Enterprise, p0et had provided a wrong information for you. WPA8630 V1 which support WPA / WPA2 Enterprise. You can see the User Guide in Page 19....

Wired two of the deco units through a switch, leaving the third one with wifi only. That's what I did on my side and it seems working just fine. Not sure if it's powerline devices...

try as the user guide http://static.tp-link.com/TL-WN722N(EU)_V2_UG_1476755166109v.pdf

nope, you can check it from Utility.

if you want a WiFi, you should connect your router and one of your powerline adapter directly. It is just an extender.

try the PLC-to-VDSL mode