TL-MR3420(Four-Address Format WDS) cannot WDS C9(Three-Address Format WDS) successfully.

You'd better try different web browers to login the web interface. Ensure the connection between WPA4220 and PC is stable enough.

TL-WPA4220 is proper in your case. Since there is no WPS button on the hub, to clone its SSID and security settings, you can install the utility for WPA4220 and login its web interface to change...

There is a tool for you to disable power-saving. http://forum.tp-link.com/showthread.php?3666-how-to-solve-some-powerline-adapters-can%92t-be-waked-up-by-some-network-adapters If the problem...

Can you provide model number of your powerline devices? You'd better check the rate using utility for powerline adapters. Some powerline devices may have high rate the the speed still cannot reach...

There are no much differences ... If you can select hardware version when you buy a new TD-8616, it is better to buy the latest version.