I bet you were smiling to yourself in a big way when the spare key worked. It is good that you have a mechanic you can trust who is honest.
Your story reminds me of the story of my dumb ass sister in law. She was "locked out" of her car because the remote key fob wasn't unlocking the power door locks. She called a locksmith to come out and "unlock " or open her car. When the guy arrived he said, "are the keys locked in the car?" She answered, "No, I have the keys right here." He took the keys from her, inserted the key in the lock and turned the key. The lock popped up and he then pulled the key out and opened the door.
"That will be $65.00 Mam.", he said. "You might want to change the battery in that remote too."
She had no clue that the physical key could unlock the door. She's a school teacher though so that might explain a lot on how she "thinks".
I like that you read between the lines, "especially nowadays!", you know exactly what I'm saying.
Thanks for great story and support,
Sult