I think you have to put yourself in the position of the workshop owner. How would you feel if something like that just happened, and the customer insisted it was your fault?
Does it have to be anyone's fault? Things will fail at the most inappropriate times - at least it really does sound like just the aux belt tensioner (a relatively easy part to replace), and surely it's either now faulty, or it's not.
If, for example, it has a worn-out bearing, you can't possibly blame that on the workshop. It just chose that time to fall apart - maybe certain running conditions as part of the MOT test. It might not be the tensioner at fault; could be a seized alternator, for example.
I do sympathise with you and agree that if someone asks you "has someone been messing with it", it does start off sounding like a blame-shifting exercise that tends to fuel your anger. I expect in that first conversation, the mechanic could have chosen better words. Mind you, you don't generally expect mechanics to be psychologists, though if they are Alfa specialists, it helps
So my suggestion - pick up the pieces and move on, sorry. Could be a lot worse. I seem to recall reading about a Lotus Elise (somewhere) that fell off a four-point hoist in a workshop, and damaged several other cars. That is obviously a different situation worthy of an insurance claim.