@mrt10001 When the device is unplugged, after holding volume- and power for long enough, it eventually boots into Ubuntu. Sometimes it takes 20 seconds sometimes it takes 60. It never loads the bootloader any more.

EDIT: ok I got it to show up with bootloader for about a second by holding power for 10 seconds as if I was turning it on as usual, then releasing when it vibrates, then volume down and repressing for another 10 seconds. Going to experiment some more.

EDIT 2: for anyone else who ends up with their device in the same state as mine: the trick was to continue holding down both buttons and click on the touch screen while still holding them down, and then I was able to release once it started the bootloader.

Thanks very much for the help! Hopefully I'll be good from here.