I hate to break it to you, but unless you store your home folder on a separate encrypted partition (which would be really inconvenient), this won't work.

FileVault 2 encrypts your entire boot volume, so there's no separate encryption of your home folder going on. If you give someone any of the passwords that unlock the boot volume, they could get all of the data off it (by using target disk mode or booting to single user mode, etc.)

Exactly. Or just enable the guest account. That's what I always do.

I assume they might want admin permissions honestly.

Having said that, prior to my recent visit (failing mbp fan) my trips had always been for failing/failed discrete graphics (about 5 trips on 2 different generations of 17" mbp) so a reinstall is inevitable because making macOS work properly after a mainboard replacemnt is more work than reinstalling from scratch

It so easy to get admin rights on a Mac anyway.

- Boot into single user mode...

# mount -uw /

# rm /var/db/.AppleSetupDone

# reboot

Reboot the machine and it walks you through a re-run of setting up the OS X set up assistant, which allows creation of an admin account.