Devices should normally switch to the strongest signal. Some have a very low threshold for doing this, effectively only changing to a different AP on the same network when they've lost signal from...

No. These multiple ports work exactly like an ethernet switch does. You can use the extra port near your router to plug a device in there.

Yes you can operate multiple 8630P on the same powerline network. You cannot however purchase one 8630P alone, you'll always have to buy the kit with an extra 8010P in it.

Very possibly, those PLCs made for China aren't compliant with your current country's regulations. Violations would be on both the PLC and the WLAN sides. I wouldn't bother.

Yes - the AV2-1200 standard is backwards compatible to 600, 500, and 200. You'll want to put your internet router onto one 8010 from the new kit, such that the 8630 has the best possible link....

The UK versions are upside down from everywhere else's, connectors on top.