If you've got everything now on your pc. Just fastboot the complete factory image including userdata and start the phone from fresh. Then just copy all your stuff back over
It looks like the partition successfully mounts every 1/20+ reboots or so. Right now it managed to mount in TWRP and I'm copying over my files before doing anything else.
Just to be clear this is the second time this is happening. After the first time I _did_ do a complete factory image flash and started fresh. Now, 2 weeks later it happened again. My eMMC might be failing...
EDIT: After the backup completed I ran e2fsck /dev/block/platform/msm_sdcc.1/by-name/userdata , it was found to be clean. I then wiped caches and rebooted and it's working. We'll see how long this lasts...