The only thing I dislike about this forum is the breed bashing that happens on here.
The neighbours shouldn't have left town with a new dog at the house - it takes at least 3 months for dogs to settle down and their true temperament to show. Leaving the house so soon after getting the dog would be very stressful for the dog. They also should have a fenced yard - for any dog, but especially an active, athletic one.
If you can't handle a large dog or you're afraid of it, you need to be honest and don't agree to look after it - that's your responsibility.
He's in a new environment - of course he's curious, and of course he's going to want to mark everything. Clearly, he was bored in the garage - it's not a surprise he ripped his bedsheets and ate the cat food. I'm shocked he didn't do it sooner.
If a Golden or a lab had done any of these behaviours (ie. the grip with inhibition), people laugh it off...but when it's a pitbull the automatic reaction is it must be people aggressive, which equals a death sentence. Can they be dangerous? Yes - but so can any dog. The breed I actually am most cautious of is the husky - I have seen far more incidents with huskies than I have with pit bulls.
Your neighbours do sound inexperienced, and even with the best intentions that is clearly a powerful dog. They need a fenced yard, a good vet to get him neutered ASAP, a good trainer, and to commit to meeting this dogs mental and physical needs...but I would say the same thing if it was any dog exhibiting these behaviours. If they can't do this, they are better off without the dog - but not because of the dog - because of the people.
I do realize that my opinion represents the minority here, but I have worked with and trained a wide variety of dog breeds, including many pitbulls and pitbull crosses. I have never had a pit bull try to bite me or show aggression towards me, which is more than I can say for some other dog breeds. I wasn't going to post on this thread...but I just came back tonight from working with a wonderful pit bull who is available for adoption. She would be a fabulous pet or working dog, but we couldn't bring her to our adoption event last weekend because she would have had to be muzzled the entire time because of her breed. She is the nicest, best behaved, most biddable dog at the shelter...but breed bias is working against her. It's not fair.