16 at most... You want to establish two powerline network? If it Okay, pair them separately. http://www.tp-link.com/en/faq-258.html

Yes, it is compatible with VDSL/ADSL/Fiber/Cable services. Broadcom chipset.

You can download the utility here. http://www.tp-link.com/en/download/TL-WPA281_V1.html#Utility Follow the guide here to change wireless settings. http://www.tp-link.com/en/faq-398.html If you didn't...

It has no such a feature. If it matters to you, you may submit your suggestion here. http://ticket.tp-link.com/index.php?/Tickets/Submit

I think you should place the extender in a proper location... It was a little far away from the router maybe... Move the extender away...but ensure that 5G/2.4G light is green... Test the speed...

What's the operating system of your PC? Download chipmaker's driver(MT7610U) and install it for a try. http://www.mediatek.com/en/downloads1/downloads/