Hi all,

After attempting to downgrade Fallout 4, I was stuck in a loop with Steam Guard. It would tell me to authorize the login and press continue, I'd get the alert on my phone, approve it, and hit continue just to get the same prompt and alert again. I did this a few times, and it did not work at all.

While following the instructions to downgrade the game manually, the depots failed to download (see images.)

When I restarted Steam as admin, it asked me to log in, and now will not allow me to log in to steam at all, so not only am I out Fallout London, I'm out all of my Steam games as well.

To say I'm frustrated is an extreme understatement. Any advice is welcome.

Not sure why you needed to run Steam as admin, but is never a good idea. I followed same steps as in guide (just executing depot download commands adding new command after previous one completed) and after quite a while all steps were done, then renamed old Fallout 4 folder and moved all depots files into a new empty Fallout 4 folder at same location.
First check if there is enough space to download all depots, for Steam log issue can try to clean cache and temp files (look for guides) and try to login again.