Timing Belt ripped after 20K, cracks all over it. Mitsubishi Galant 2003.

Timing belt, tensioner, pump, etc (Timing Belt Kit) were replaced on my Mitsubishi Galant 1 year ago. After 20,000 the timing belt ripped. My mechanic got the belt out and showed to me - it has cracks all over. He says it must have been defective to begin with, probably had been stored way too long before I bought it. I went to the seller (all parts are aftermarket) and showed them pictures of the belt. Their technician claims it is cracked like that because its tension after installation was too high. My mechanic says, that with hydraulic tensioner there was no way to overtighten the belt.
I do not expect to be able to figure out for sure who was at fault here. But can someone please help me at least to understand better a couple of things:
1. If there is a hydraulic tensioner, is it still possible (or probable?) for a skillful  mechanic to overtighten the timing belt? If yes, cracks all over the belt are expected?
2. Are there any limitation on how long a timing belt may be stored after being manufactured? If it has been stored somewhere for 10 years, for example, is it plausible that it looks fine, but after being properly installed it cracks and rips?
3. Any other thoughts about this situation? And I was told already that it is not a good idea to buy an aftermarket timing belt, thank you :-)
Thanks everyone!
P.S. Oh, and I got lucky, the engine is not damaged.