500 mb/s is the theoretical maximum on a physically perfect connection. In real life, nothing is perfect, so nothing wrong here.

Probably the WPA adapter has a different IP address now. Check the DHCP logs in your new router.

I'm setting my secondary WiFi APs to obtain an IP address via DHCP - and set the main router's DHCP server to assign them a static address. That way you don't end up having static IP addresses...

Sort of correct - as long as there is sufficient crosstalk between the two rings, they'll work across. If not, you can still add a phase coupling device to your central fusebox.

The AV500 kit should happily connect to newer ones. I'd recommend you go with the AV1200 series for best performance with well matured firmware. [Besides, AV1200 is basically AV600 on two channels,...

Set them up to use non-overlapping channel frequencies, at least four numbers apart - e.g. channels 1, 5, 9, 13 in a four access points setup. If your environment lets you use wide channels, use 1...