There is an instruction here. Follow the guide to check your IPv6 settings. http://www.tp-link.com/en/article/?faqid=857

It is a problem related to Vista OS. Change security settings on the router. If it is WPA-PSK/WPA2-PSK, change it to WEP or disable security. Remove the network profile and reconnect. If you don't...

As I know, currently, all TP-LINK powerline adapters are compatible with each other. To guaranty data transfer rate, you can still buy an AV500 powerline adapter.

YES！ You can run the utility for WPA4220 and login its web interface to change Wi-Fi name and security settings.

Configure static routing on WR841N to fix the issue. http://www.tp-link.com/en/article/?faqid=560

Restore the device to factory defaults. And in default setttings, both 2.4 and 5GHz concurrently option is selected, right? How about 5GHz status on Status page?