The bolt holding the adjusting screw probably came loose after decades of use. Last one like this that I serviced, I used a dab of Loc-Tite to hold the bolt in place. It's still toasting!
Oh, Quebecois, don't be so sensitive. Also, you should know that my grandparents were from Trois Rivieres, Quebec. So I would be quite unlikely to mock you about your language.
However, perhaps this is the toaster you're looking for?
Knowing when to quit is a huge part of any challenging repair project. You gave it your best shot. Another vintage toaster will come along for your daughter.
I'm not ready to give up on my short circuiting vintage Osterizer Cyclomatic blender since there are no broken parts -- yet.