1998 Volvo S70 Timing Belt Vs. Tensioner

Engine problem
1998 Volvo S70 Engine Size unknown Front Wheel Drive Automatic 41000 miles

Car engine failed and car had to be towed to dealer. Dealer said it was probably a broken timing belt, and if so, they would need to replace the belt plus the tensioner (as these are two integral parts).

This is what they found:

Belt had 'jumped' and was wrapped around the tensioner. Tensioner and timing cover were broken.

They replaced the belt and tensioner, then determined there was also valve damage.

Problem: Per my mechanic there is no DEFINITIVE way for them to know which came first, the belt jumping or the tensioner breaking. He made an educated guess that the belt jumped based on the fact that the car only has 41000 miles and the car is a 1998. He suggested belt may have stretched or loosened. He said there is little likelihood of the tensioner breaking on it's own (at such a low mileage) and reports no rusting, corrosion or other cause for the tensioner to have broken other than having been broken as a result of the belt whipping and winding around it.

My question: I want another opinion on this because my warranty covers the problem if the original problem is the broken belt but not if the original problem is the broken tensioner.

Needless to say, the inspector for my powertrain warranty (who went out AFTER the belt and tensioner were replaced) has decided to deny this claim and is insisting the problem has to have originated with the tensioner.

Any thoughts or ideas on how I can get a DEFINITE answer to which came first? The belt or the tensioner?

Please advise asap.

Note: a donation has been made.