Hi @What Now?,
That is a terrible situation, possibly the worst-case scenario from this bug. You are in a complete deadlock.
You are correct:
You can't log in to Windows.
You can't use the PC's WinRE (mouse/keyboard bug).
You can't use the USB Media's WinRE repair options (because BitLocker is blocking them).
There is one possible path left. This is advanced, but it is the only way.
The key is your 48-digit BitLocker Recovery Key. You must have this. You can find it by logging into your Microsoft account on another PC or your phone: https://account.microsoft.com/devices/recoverykey
If you have that key, follow these exact steps:
Boot from your Windows 11 Install USB (as you have already done).
On the language/keyboard screen, click "Next".
On the "Install now" screen, click "Repair your computer" (or select "Repair this PC" at the "Setup options" screen).
This will load the clean recovery environment. Your keyboard/mouse will work.
Go to: "Troubleshoot" -> "Command Prompt".
A command prompt window (starting with X:\Sources>) will open.
This is the most important step. You must manually unlock your C: drive. * First, find your Windows drive letter. (Type diskpart, then list volume, then exit). Let's assume it is D:. * Now, unlock the drive using your recovery key: manage-bde -unlock D: -RecoveryPassword YOUR-48-DIGIT-KEY-HERE * (Replace D: with your actual drive letter, and YOUR-48-DIGIT-KEY-HERE with the key.)
Type exit to close the command prompt.
Now, you are back at the "Advanced options" screen. Because the drive is unlocked, the other options should now work.
Try "Uninstall Updates" (to remove the latest update).
If that fails, try "System Restore" (to go back to a time before your PIN stopped working).
If you do not have your BitLocker Recovery Key, then unfortunately, the only option left is a Clean Install, which will erase all your data.
I know this is extremely complicated. If you are not comfortable performing these steps yourself, I strongly recommend asking a tech-savvy friend or a local PC repair professional for help to avoid making the situation worse.
(I discuss many of these recovery scenarios on my technical blog here - the site is in Japanese, but translation may help):https://windows-waza.com/how-to-change-back-after-changing-win11-administrator-account-to-standard-user/#index_id1