If FileVault is enabled, booting into single user mode requires a password, so neither of the methods for resetting the password in single user mode work.
As others have mentioned, resetting the login password doesn't reset the password of the login keychain. I tried resetting the login password a few months ago. I could access most files normally, but not my account in Mail.app or auto-filled passwords in Safari. But I could access my Gmail account from Safari because I had set it to log in automatically.
The login password (but not the password of the login keychain) can also be reset with an Apple ID. There's a checkbox for allowing that when creating an account. I think it was checked by default. The option can be disabled later in the Users & Groups preference pane. If the option was enabled before you turned FileVault on, you cannot disable it without turning FileVault off and back on. See Michael Tsai - Blog - FileVault 2’s Apple ID Backdoor.
1. Are all user accounts still present?
Yes. And all of their passwords can be changed separately, or from the Users & Groups preferences after logging in to (or creating) one adminstrator account.
2. Are all keychains still present, or are they reset or cleared as part of the password creation process?
The keychains aren't removed, but the login keychain isn't unlocked automatically after the login password is reset.
3. If the keychains are still present, does the (new) administrator have access to the contents of existing keychains.
Not without knowing their passwords.
4. Are any files (other than keychains) erased or reset as part of the process?
I don't know, but you can access most files normally.
The response I got from Apple Support:
For many repairs, as well as quality checks after a completed repair, it is necessary that our technicians can boot the Mac. If FileVault is enabled, that means that we do need to note the password for the time of the repair.
A viable alternative is to wipe the Mac’s hard drive or flash storage prior to the repair and restore from a backup afterwards.
So either prepare to hand off your password to them, or empty your hard drive before giving the macbook for repair.
Best Answer
If FileVault is enabled it is impossible for anyone without the FileVault password to access the data on your computer including any services like Notes which you may have.
Please note that what you have just done on your partner's laptop may be illegal and I would caution you that this is a public website where you may just have admitted to a crime depending on your location.