Left iPhone in a taxi a few weeks back!

I left my iPhone in a taxicab the day after thanksgiving, with 3% battery remaining. I'd paid cash, and there are a million different cab companies it could have been (I've called the major ones and the Boston Police Department hackney service).

I immediately noticed it missing and used Lost Mode. I got two notifications saying it was found-- first on Prospect street in Cambridge, then 20 minutes later on Mount Auburn street. Then the battery died and it's been radio silence since. Currently, location history is unavailable, as the phone is offline. It clearly hasn't been turned on-- or if it has, someone has bypassed it's connection to the internet. I did not have a passcode on my phone. Also. The phone had a crack on the screen-- I'm hoping that deters someone nefarious from selling it.

The data on the phone is more valuable than the device itself.

A few questions I have

1. I assume the notifications I got while it was still charged were simply that it had been found via GPS, not necessarily that a person said "I found it".
2. Did putting my phone in "Lost Mode" mode effectively lock it? If so, to what extent?
3. If I change my (AT&T) cell service to another device, will my lost phone still display the last "Lost Phone" message it had, if it turns on?
4. What are my best bets for recovering? I have some sentimental, non-backed up photographs and audio recordings from Thanksgiving, which is likely my grandfather's last.

Thanks for any and all advice!