Alright, I finally got it resolved. Here's what I had to do:
- Log into laptop over the network via Remote Desktop.
- In Settings, use the "Clear TPM" feature and restart when prompted.
- During the reset, enter the UEFI BIOS and DISABLE the TPM module.
- Save & exit BIOS, continue booting.
- Log into laptop again over the network via Remote Desktop.
- In Settings >> Accounts >> Your info, click "Sign in with a local account instead". (This confusingly-named feature does NOT delete your current account, it just unlinks your online Microsoft account from it. You will not lose any apps/settings/data by unlinking.) Enter a new temporary password for the unlinked local account when promoted to.
- Restart the laptop.
- Log into the laptop using the local account password. Hooray, you're in!
- In Settings >> Accounts >> Your info, re-link your Microsoft account, and set up a PIN when the process asks you to do so.
- Restart the laptop again.
- Now I can finally log into my Microsoft-linked account via the PIN again. Hooray!
- Restart the laptop again.
- Go into the laptop's UEFI BIOS.
- Re-enable the TPM module.
- Save & exit UEFI, continue booting.
- Done, all fixed... until the next time this same bullcrap happens again for no good reason.
This requires that the machine already be successfully on the network and configured to allow incoming Remote Desktop connections, and it also requires you to have another PC, tablet, or smartphone on the network so you can connect to the problematic PC. So these steps may not be able to help everyone. Hopefully they help some folks though.