Locked: Mechanic damaged my car, what can I do?

My wife drove our 2013 highlander to a mechanic for service yesterday. The same mechanic did the service for this car last year. So nobody else touched this car for like 2 years except this mechanic. (the lockdowns and virus has caused less frequent service).

I checked the maintenance schedule before my wife went. just oil and air filter change. The mechanic told us to do gear oil change. we asked why the official maintenance schedule doesn't say, he said as per inspection. It's ok the car is more than 8 years old anyway. 140 dollars for gear oil change.

My wife came home in the evening and I drove the car to LCBO. As soon as I sat in the car I felt something wrong. I was adjusting the seat and immediately found the seat panel came out (as the first picture). I am pretty sure the panel had no problem last time I drove this car. (Why? funny thing we went shopping last sunday, I don't like shopping so I waited in the car. I adjusted the seat back and forth to make myself comfortable , so I am pretty sure the seat panel was totally fine last time I drove it).

Then I checked the seat panel, the plastic piece snapped as the second picture shows, that's why the panel came out. since it was good last time we drove it, we called the mechanic. He simply said he didn't touch anything and didn't damage our car. I scratched my head and thought wow, so the panel happened to break down on the way to service? what a luck!

This morning I checked the car again. I found something. As you can see in the picture 3, the brand new mark on the screw! It's obvious somebody tried to unscrew it and left the mark their. Otherwise how this screw hidden inside the panel has this new mark on it?!

I called the mechanic again to ask why he even tried to unscrew the seat panel as we only did some oil change?! He insisted he didn't do anything. And said he would look into it and fix it for free if there is no cost. There is no easy fix unless replacing the panel because the plastic piece already snapped! More than this, I am really curious why the heck he even tried to touch this part! Maybe tried to take some part like module like I heard some bad mechanics did?! wow, I am gonna stop going there for sure. (I have an appointment next week for another car). What can I do now? Is there any government department I can report this kind of things?

Thanks a lot in advance!