Lost access on my own computer?

Ok, thanks. The only common thread that emerges from this list of actions is that, for some reason or another, your user account has an issue. This was verified when Apple Support had you create a new user account and the issue disappeared.

Also the fact that, even with your current user account, that running your Mac in Safe Mode, still did not eliminate the issue. Again, this would lead me to believe the issue is with that account ... and troubleshooting exactly what in that account would be the culprit(s), would not be an easy task.

My only suggestion, at this point, would be to elevate the working user account to the administrator level, log out of your current account and into the new account, and finally, delete what was the current account. Then restore data from your previous account at your risk.

So that we don't try offering potential solutions that was already offered by the Apple Support team, can you please provide us with what those were?