This was very scary - without factory images to reflash it with, I thought I had bricked my device! I did a fastboot getvar all, and saw: As it turns out, none of them did (and, in fact, they all blanked the screen and went back to having the USB interface enumerate as fastboot - how could that be? does anyone understand what could cause fastboot to reject a kernel before even trying to boot it?), but when I was done for the day, I had another disturbing event: when I tried to boot the device normally, it would simply sit there at the fastboot screen, and in fact, when I tried to boot into recovery, it would still just kick me back to fastboot.

Yesterday, I was messing about trying to fastboot boot other MSM8998 TWRP boot.imgs, with the hopes that the device-trees on some of them would be close enough to get far enough in boot to bring up the USB port (even if they didn't mount anything or bring up the screen).