[Nexus 5X General] DrSteveBrule1:
This is excellent information to have for the future. I would never have thought it so easy to bypass the security, though I guess you still need the passcode to get that far in TWRP so as always, a system is only as strong as its weakest link.

I restored one of my old backups, then factory reset. I made it into the system, did the initial setup, then set up my security exactly as it was before. My backup is currently restoring again, at which point I'm going to try updating again. I'm now feeling fairly certain that the issue I had with the security screen came from my successful flash of the updated CM snapshot. I wanted to take a quick peek to see if I noticed any immediate changes, so I completed the initial setup. I skipped the setup for a security key. It must have replaced my above mentioned password.key and pattern.key files, but when I restored my full backup, it was expected a pattern on a different size grid than what was stored. There was no possible way to enter the correct pattern.

I'll continue updates as I try to get this working, as I pretty much have to get it done today.

Thanks again for your help!

Update 1: My idea of factory reset -> set up security -> restore backup, like the goggles, did nothing. Your suggestion, however, worked like a charm (though I just appended .old to the password and pattern files)! So now my current state: I'm back to my working backup from 8/1 (which is running the later-than-anticipated nightly from 7/26... neat!). I'm now about to try dirty flashing the 8/16 snapshot. Cause what the hey hey. Why not, it might even work this time.

Update 2: It did not work this time. So I reset to factory, and flashed the snapshot again, and then wiped cache and dalvik. It appears that I am again stuck on the android boot logo, which is confusing. Really wanted it to just work, so I could restore my data partition and delete the password and pattern files again, and hopefully be done. Going to try to shift focus onto another project for a bit and see if I come out of this bootloop by miracle?

Update 3: Did not work. Tried some other things, which honestly is just a more of the same. At a loss.

Update 4: I realized that on my factory resets, I have been neglecting to wipe system. So after doing "Advanced Wipe" and selecting everything but... media(?) (as to maintain my data), a fresh flash had no issues with booting. I then went back to TWRP and restored my data only. Next, I deleted the password.key and pattern.key mentioned in the above post. Finally, I will reboot my phone to a functioning system with all my data and apps. A caveat to this is my system apps have not transferred over, and I will have to flash a new gapps.zip. Of course, this will have to be done before the phone is booted otherwise a ton of stuff won't work without the google blah blah framework. I had an "Unfortunately, setup wizard has stopped" infinite message still after this. I mounted system in TWRP mounts then used the file browser to delete /system/priv-app/SetupWizard and the apk inside. My google services all still seem to be working, and I managed to install an app from the play store. I'm not sure what the adverse affects of this may be, but I think at this point, I am too tired to go on with this. Time will tell if everything works out. Also looks like I have to reflash Xposed.