I think the one you have must be a Chinese product, while the easy setup assistant is English version. so maybe that's why the English setup assistant can not recognize this model.

Maybe you can try to change them back to Auto and reconfigure it to static. select the PC’s MAC address and change the Status to Auto.

So she has a WPA281KIT and a WPA271? yes, they can all work together. the wired plug connects to the internet, and the 2 wireless homeplug can be put in the different rooms.

the 310U can only be used by the MFP and storage server installed in your computers. so you just contact tp-link support to get the MFP and storage server for windows 8.

probably this model does not support unifi. So you might consider to change a wireless router that support unifi like WR1043ND, WDR4300 or WR841ND.

is there a router in the network to share the internet for the devices? what kind devices are there connected to the two PA251s? OR how many devices do you have including modem, router, and 3...