New (old) engine or rebuild.... And fault?

[BurnsieUK]

So, to shorten the story.

My aux belt went one day.
Drive the car to a garage.
They fit a new belt.
Belt breaks the next morning on way to work (well, I made it 5 meters!).
Garage can't work out why. Keep talking about timing belt being old (was due to be replaced at 100k miles but it had clicked over 100k miles between services).
Car starts but with misfire
Old garage pays for a new specialist garage to have a look who confirm an old timing belt, that's it's out a bit
New garage suggest an old second hand engine would be cheapest fix
No one can find a second hand engine
New garage probably going to have to strip the engine down to see what's what.
(I am aware that there has been stories of aux belt causing issues with timing belt, even though timing belt is encased).
Local independent Volvo man (who was too busy to look at my car) says it doesn't matter how overdue the timing belt was, if it worked it worked. If it didn't, it didn't!

First question from me...
Anyway I could find out what caused to timing belt issues? I'e I drove my car into the garage when the first belt went.... I couldn't do the same after the second one went.

This has been ongoing since Jan due to Covid for a start