Yes, it works. In addition to what has already been said, it is important never to connect to a multiple plug. A multiple plug/plug strip on the Devolo, on the other hand, works. I had a lamp with a dimmer on a socket that was connected to the one on which my Devolo is connected. That didn't work either, I had constant interference.
Yes, this works. If the units are not connected to the same fuse, problems may occur as already described. Try pairing the two devices in the same room first and then installing them in the definitive locations. This helped me.
Yes, it can be extended as much as you like. The bigger problem is when the circuit is not connected to the same fuse; then only very poor or no transmission occurs.