A couple guesses about the ending ...

Guess 1. Whatever caused the blackout is an active force. Whatever it is, it's actively retarding electricity (possibly other forces too?) and by activating the pendant it blocks that force (kinda like a cell phone jammer?). This would assume she was running things off of battery power since it didn't show her firing up any power supply like a generator.

Guess 2. The blackout caused a permanent physical change in matter that retards electricity. The pendant temporarily restores the natural properties of matter in its direct vicinity and even causes electricity to flow when no power supply is available.

In either case, due to it's size it has a limited effect.

Regarding the communication network, I'd assume it is operating on some sort of radio technology because the existence of any sort of wired connection is simply not plausible. It's also possible that this inner-circle of conspirators have some sort of blackout resistant central command or satellite since it seems like the pendant holders were involved in the cause of the blackout.

As a side note, was I the only one to think that it's not exactly subtle to hide your secret communication centre behind the only interior door with 5 locks on it? And then, after unlocking it, she leaves the door wide open while inside?