Ethernet over the powerlines is always an "iffy" proposition.
That is because there are too many variables to be able to make any kind of accurate prediction about how the network might function. Some examples.....
Bandwidth is lost with distance over the powerline
Bandwidth is lost as the signal changes from one circuit to another
Noise on the AC powerline may affect performance
Filtering in the powerline adapters may not allow routers to function correctly
The bottom line....you won't know how well the system will work until you try it in your home. For that reason, it would be a good idea to understand the store's return policy in case things do not work out as hoped.
Let us know how things are working there when you can.
does this configuration work, at least in theory? I am not connecting the AEBS to the extender AEBS/AX using ethernet, but rather using the powerline.
Yes, this is the theory behind powerline. But, you will need some short Ethernet cables to connect the Apple devices to the powerline adapters.
This may be a dumb question but I'm not sure how these things are intended to work, and all the literature i've read on them discusses hooking them up to individual network devices, like game consoles or wi-fi'd DVD players.
If you look at the documentation with the adapters you will see that you would connect a short Ethernet cable from one of the LAN <-> ports on your main router to the powerline adapter, which is plugged into an AC socket near the main router.
On the other end of the house, you would install another powerline adapter in the AC socket and connect another short Ethenret cable to the WAN "O" port on the Apple router at that location. The Ethernet cables can be any length that you need.
If you are not sure how to configure the Apple routers, this support document will provide the details: