So they're still running on the old network key. Since the TP-Link kits don't use a secure key out of the box, I suggest you start over. Factory-reset them all, plug just one of them back in. Now...

The general answer is Yes - all the HomePlug AV standards are compatible, all the way from 200 to 2000 series. Slower connections will eat into the bandwidth of the faster adapters obviously.

I'd go with Devolo's 1200 series, if these are available in your country. They have well designed and robust hardware, rather excellent warranty handling if something should fail, and an immaculate...

Yes that'll work. I'd save myself from the hassle though, and just buy a product that has (1) an auto-standby feature that actually works and (2) a WiFi on/off button on the WiFi enabled head unit.

As it says on the box: Just PLC, no WiFi.

When there's no W in the product code, then there is no (W)ireless functionality.