1. Yes, same wires for drawing power and feeding back into the building.
The regen drive must be phased correctly to operate. This is accomplished at installation. This is probably what you mean by "synchronize"

2. If a heavily loaded elevator is moving down, or an empty or lightly loaded elevator is moving up the regen drive is producing power. However, if the power goes out when the elevator regen system is producing power, the elevator will quickly stop due to the loss of power and the brake setting. The motor will not be spinning anymore so no more regen.

3. In most regen situations, the electrical load will be distributed locally, and the power meter will just slow down. If the only load is the elevator operating in regen mode, power will go to the electrical utility grid. If the building meter does not have the capability to turn backward, power will still go to the grid, but the building will not reap any cost benefit. This scenario would be a very rare occurrence anyway.

4. Here is a good link to the info you want:

http://www.elevatorbooks.com/Content/Site125/FilesSamples/179592pdf_00000088467.pdf