Help needed with Moto X XT1053 corrupted partition

I need a lot of help with my Moto X, which is stuck booting. I can get to the bootloader, but no further.

Until a few days ago, I was on stock 4.4.4 (rooted, unlocked bootloader). I decided to try out the modified stock 5.1 posted on this thread . I had no problems making a backup on TWRP before flashing, but after I flashed 5.1 and went to make a backup, I got stuck in a boot loop.

I had to factory reset from TWRP and tried again, same problems (boot loop whenever I made a backup, and no access to TWRP folder from the OS), so I downloaded the full firmware (TMO_5.1_LPA23.12-15_15_release-keys_Retail_US) from this thread and tried to flash manually using ADB and fastboot, following the instructions in the ReadMe file in the firmware.

Everything was fine until I tried to flash system.img, when this happened:

D:\Phone\MotoX\root>fastboot flash system system.img
(bootloader) Variable not supported!
target reported max download size of 805306368 bytes
Invalid sparse file format at header magi
error: write_sparse_skip_chunk: don't care size 603230508 is not a multiple of t
he block size 4096
sending sparse 'system' (785522 KB)...
error: write_sparse_skip_chunk: don't care size 603230508 is not a multiple of t
he block size 4096
error: write_sparse_skip_chunk: don't care size 603230508 is not a multiple of t
he block size 4096
OKAY [ 51.425s]
writing 'system'...
(bootloader) Sparse image is malformatted
FAILED (remote failure)
finished. total time: 88.818s

I tried a couple of times, but the same result. So I tried the instructions for manual flashing in this post, and got a different error:

D:\Phone\MotoX\root>mfastboot flash system system.img
load_file: could not allocate 1438613804 bytes
error: cannot load 'system.img'

I'm stuck now and have no idea what to do. It's the exact same file, so I'm not sure why it's being read as two different sizes. Also I'd like to be able to use my phone again! Can anyone help, please?