[arrrghhh]

1. Do you have the original data.img...?

2. /data/system/dropbox. Look for the SYSTEM_LAST_KMSG files. These are basically kernel messages, and really the only record from a reboot.

3. Keep the battery charged? Not sure what you want here. I've honestly never experienced what you described. I have a feeling it was a coincidence that it rebooted - because every time my battery has outright died, I just check the phone and it's dead. No reboot to WinMo, just unresponsive.