Locally, can you write files? Ensue anti-virus software and windows firewall are disabled.

Did you read canuck's “long story”? The manufacturer name can be detected or not, which depends on the software itself or the version.

TL-PA451KIT and TL-PA4020PKIT are compatible with each other indeed. If PA451 and PA4020 have been paired and they can communicate with each other, they should work as your expection.

In #1 and #3, what you said are different. So two buildings are in the same electric circuit or not? If not, you can purchase a range extender, like WA850RE.

Does your router has such a feature?

Use different channels for the root router and powerline extender. Security settings should be same to each other. What you are expecting sounds like seamless roaming. Unfortunately, WPA4220 has no...