Hi Simon. I would Expect the coil feed to come from the Main Relay. whether its seperate or built into the underbonnet fuse box.

I would think its lost all the lights and windows etc due to going into Economy mode which they do after the battery starts getting low due to it being cranked over and it not starting. ???? It will come out of Economy mode as soon as you manage to get it started.

I have had a couple where the BSI has lost comms and stopped the vehicle from starting, usually losing all info stored in them and requiring reprogramming. ( but not able to do so due to no comms )
Afer the BSI has been Disconnected for a couple of days, I have plugged them back in, reprogrammed them and they have worked fine afterwards. ( Usually after i have bought a new module and end up with it stuck on my shelf )

As for it being a Dealer job, NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

Only warranty jobs are dealer Jobs.