What is accessible to an attacker who has reset my administrator password?
If someone has your administrator account, everything on the machine is accessible to them. People in IT Security circles say "Physical access = Total access".

Keychains can only be unlocked with the keychain password.  If an account password is changed/updated without being logged into the account AND having the relevant keychain unlocked, the keychain password doesn't change.

Meaning, someone could get access to your account perhaps, but not your keychain, without the original password.  The account and keychain passwords fall out of sync with one another.

FileVault protection similarly, will remain in place until unlocked using the correct password.  I don't believe having root access on the machine allows you to unlock the keychain or the file vault account without the relevant passwords.. that's the whole point.

Of course, theoretically a hacker with godlike skills, patience, and knowledge could manually edit the relevant passwd files to be whatever he/she wanted it to be (if they had root access), but the probabilities of this happening is somewhere between zero and nil.