After failing to update and launch VS installer, I decided to reinstall Microsoft Visual Studio.
Neither setup.exe nor InstallCleanup.exe worked for me, so I decided to delete all files in the Microsoft Visual Studio folder, leaving nothing but the empty "Installer" folder which cannot be deleted, because apparently it's being used by a program I cannot spot in the task manager. How do I gain access to delete or modify Microsoft Visual Studio\Installer so that I can reinstall VS?
I managed to do it using an app named "lock hunter". It's a free software found on https://lockhunter.com. after installation it adds an option named "what is locking this file?" when right-clicking any file or folder in a file explorer. It will open a window where all processes using that file are listed, and you can terminate them from there.
Turns out Installcleanup.exe was still trying to clean MSIs, even though it would never finish and was just holding an empty folder hostage. By ending that process I was able to delete "Microsoft Visual Studio\Installer" and reinstall the program, which thankfully worked perfectly this time around.