They have a RAT (ram air turbine) that deploys automatically under specific conditions. It’s basically a turbine providing electric and hydraulic power. It was almost certainly deployed on the accident flight. It will only power the most critical equipment, though. Possibly, that does not include the ADS-B transmitter (which broadcasts position and related data).
Yes and many pilots being walkie talkies in GA as backup. Not sure if airline pilots do this though. And its kinda hard to root around for it and fiddle with it while trying to keep an unpowered jetliner in the air. They're more for emergencies where the radio is the only problem.
By the way, the age old rule is "Aviate, Navigate, Communicate" in that order of priority. So it could be they just had their hands full with the Aviate part.
We know it called mayday and then lost communication. It also stopped transmitting GPS data.
Looking at this it likely lost all electric power. The electric power comes from generators driven by the turbines.
If you lose both turbines you lose electricity. You also lose the hydraulic system so you can not get in the gear or change flaps.
Occam's razor checks out.