In the last few weeks I have constant connection drops for one of deco units. Recently I figured out that it happened because the third unit was sending request through this problematic one. I have to restart the network when it happens but it is very frustrating!

I would expect that it works well.

One more thing, is it possible to enforce usage of powerline over wifi?

Sometimes one unit is using powerline and another one is using WiFi to the main unit. Speed is much better for the devices that are on powerline.

Has TPLink gave up on P9 or we can hope for some new firmware where we could use WiFi+Powerline together?

Cheers!