I really do not get what you are asking me exactly, but here is some info that might help clearing things
XAP 800 provides 12 inputs & 12 outputs (Ap800 provides 8 ) you can stack units together to get more inputs...
Good to know that each speaker will take one input so my 5.1 takes 6 inputs and 6 outputs... TV out is connected to sound card line in so it is part of the 5.1
If you have way too many inputs you can mix them with a simple mixer made from few resistors, or high end one (in you scenario home theatre reciever serves as a high end one)
The reason I pass all speakers through XAP is echo cancellation, in other words VC will not hear back the movie through the mic ...
Now the 5.1 sound is coming from external USB card, and I have 2 coming from my PC internal card...
Basically XAP800, is a matrix mixer, which means you can route any input to any number of outputs... So the scenarious are limitless ... I highly suggest that you download the manual to fully understand the possibilities ...
The main reason I went with XAP800 not ef2280 is power supply ... I hate those things as the always end up cluttering space and they are always a pain to manage ... As for other features I believe they are very similar
XAP 800 as well provides serial interface (rs232) for control so you can send commands, like mute or change output from a PC / I have no Vera , so I can't tell if you can use it to control XAP 800
As far as how many sound cards you need, it is basically the amount of various streams that you will need to play at the same time ..