While I am not one for engaging in law suits, I would be having a chat to one of the compensation lawerers.
While the slope may have been too steep and there may have been some contributory negligence on your part if nothing else you should get a new mower properly serviced .
Do not uder any circumstances put a spannar on the mower as it is now evidence.
After you have talked to the lawer ( attorney over there ? ) take his advice.
Sears are good people and they stand behind everything they sell.
If you don't want to get the law involved then ring Sears and tell them what happened.
Tell them you have sought technical advice and have been led to believe the mower is defective and may have caused the accident.
Sears will send some one out to look a the mower & your yard, but I would strongly advise talking to a legal person first if for no other reason that you will then know exactly where you stand.
Just remember , if you end up seeking compensation, once the mower is out of your hands it is no longer evidence.
So if you would be happy with a new mower I am sure Sears will be more than happy to replace it with a new one.