A Windows keyboard sometimes doesn't accept early boot keyboard shortcuts, or could work with different key combinations.
The Windows and Alt keys are usually reversed before the login screen or before getting to the desktop.
Try Windows-S. If that doesn't work, try Alt-S.
If neither of those work - try a different wired USB keyboard.
So, you don't know the password to either the "Estelle" or "Local Administrator" accounts?
Here's another link for changing password from single user mode. When it says to enter username, then you would enter the exact user account name that you want to change password. On that iMac, either Estelle, or Local Administrator. Doesn't matter which one, when you can log in to one admin account, then you will be able to change the password on the OTHER account, if you want to do that.
First challenge, is to boot to Single-user mode. hopefully, changing to a different key combo will do the trick.