Any question answered...

Smartphone batteries don't, but I used to get nearly a week out of my Bluetooth equipped Nokia 2730. What do you need to run on the phone for the Garmin to use it as a modem?

Bluetooth I guess to talk to the Garmin and then 2G or 3G (maybe GPRS?) for data?

I have a Sony that might fit the bill but I'd probably have to get a new battery for it and then it's still relying on a cell signal which was non-existent during the ESCA 24.

Spot tracker works without mobile signal but is a subscription service.