1. With WDS working, the computer connected to the N750 via wired should have Internet Access. If it is not working but wireless is working, try to restart the computer. 2. It depends on you. You can...

Yes, you can. They are compatible with each other. TL-PA251 has an AV pass through function.

It proves "that Engineer was wrong". Here is also an PDF file shows how to view in different OSs http://www.tp-link.com/app/usb/how%20to%20use%20local%20storage%20sharing.pdf

No DHCP from TP-LINK. How about the connections between the router and computer? MAC needs to be connected to one of the yellow ports and modem to blue ports and then log into the settings page.

Yes,it can. Once configured TL-WA830RE as an extender, the LAN port can be used for the TV and it also can extend the signal of the wireless broadband router. It worth its money.

If there is a screen shot of the error, that would be more visible to us about the error and what is going on.