[X 2014 Q&A, Help & Troubleshooting] unfnknblvbl:
Right, so here's what I did.

1) the 5.1 OTA never worked for me - would always fail during flashing for some reason. So when I found the above post, I flashed the "VICTARA_RETEU_XT1092__5.1_LPE23.32-25.1_cid7_CFC.xml" package with the following commands:

mfastboot flash partition gpt.bin
mfastboot flash motoboot motoboot.img
mfastboot reboot-bootloader
mfastboot flash boot boot.img
mfastboot flash system system.img_sparsechunk.0
mfastboot flash system system.img_sparsechunk.1
mfastboot flash system system.img_sparsechunk.2
mfastboot flash system system.img_sparsechunk.3
mfastboot flash system system.img_sparsechunk.4
mfastboot flash system system.img_sparsechunk.5
mfastboot flash system system.img_sparsechunk.6
mfastboot flash system system.img_sparsechunk.7
mfastboot flash modem NON-HLOS.bin
mfastboot erase modemst1
mfastboot erase modemst2
mfastboot flash fsg fsg.mbn
mfastboot reboot

Then seeing as it didn't want to work, and from prior experience with TWRP being annoying, I then flashed the recovery with "mfastboot flash recovery recovery.img"

Then came the bootloops. Now, I say "bootloops" but it was more complicated than that. What would happen is the boot animation would load up, then once the screen "dimmed" it would turn off. As I didn't clear userdata the first time, it would do the whole "android is upgrading x/65749823562743 apps" message... then reboot after the last one, and start again from the start. I cleared cache and userdata and then something else happened; after the screen dimming, it would turn 'off' but apparently still load. Every now & then, the screen would flash on for just an instant. Long enough for me to see some kind of error, but not long enough to find out what.

2) Giving up on this, I re-flashed the 5.0 "RETEUALL_XT1092_5.0_LXE22.46-19_cid7_CFC.xml" package with the following script:

mfastboot flash motoboot motoboot.img
mfastboot reboot-bootloader
mfastboot flash boot boot.img
mfastboot flash recovery twrp-2.8.6.0-victara.img
mfastboot flash system system.img_sparsechunk.0
mfastboot flash system system.img_sparsechunk.1
mfastboot flash system system.img_sparsechunk.2
mfastboot flash system system.img_sparsechunk.3
mfastboot flash system system.img_sparsechunk.4
mfastboot flash system system.img_sparsechunk.5
mfastboot flash system system.img_sparsechunk.6
mfastboot flash system system.img_sparsechunk.7
mfastboot flash modem NON-HLOS.bin
mfastboot erase modemst1
mfastboot erase modemst2
mfastboot flash fsg fsg.mbn
mfastboot reboot

Nothing remarkable there, although I notice that I didn't flash gpt.bin. Can't think why I didn't do that right now, tbh

Anyway, exactly the same issue occurred!

3) not having the patience to deal with that **** as it was nearing 3am, I flashed 4.4.4 "TMO_21.21.42.victara_tmo.tmo.en.US" with the following script:

mfastboot flash logo logo.bin
mfastboot flash boot boot.img
mfastboot flash recovery recovery.img
mfastboot flash system system.img
mfastboot flash modem NON-HLOS.bin
mfastboot flash fsg fsg.mbn
mfastboot erase userdata
mfastboot erase cache
mfastboot reboot

...which worked! The phone started normally and all seemed good. I was pretty chuffed.

Then, without thinking I accepted the 5.0 OTA when I awoke all bleary-eyed this morning, and she died. And here we are.

Motorola support seems to think I should return it to them, so they're coming to pick it up on Friday morning. heh.