No, Windows 10 Pro cannot be activated using a Windows 8.1 key.
No, Windows 10 Pro cannot be activated using a Windows 8.1 key.
No such thing as a free upgrade unless you specifically kept the Get Windows 10 app installed. The offer for Windows 8/8.1 none-Pro to Windows 10 Pro was not valid, so you should be fine with Windows 10 even after the free upgrade promotion ended.