High oil pressure

Had a Mitsubishi Montero Limited 3.5 L to the dealer for a timing belt, valve cover gaskets, tube seals, spark plugs. Car had 128,000 mi. On it. Less than three weeks later car lost all the oil and the engine is now toast. Dealer decided that the oil filter which had been on for 3000 mi. Was incorrectly installed by a oil change place, and that the gasket had come out of the seat on the casing which caused the oil loss, and they were off the hook because they did not change the oil. Had the oil and filter examined by the manufacturers oil tested fine, but the filter, (Purolator) had it's casing distorted by abnormally high oil pressure. If you compare the used filter to a new one you can easily see that the top was flat, but is now convex, and that is why the gasket failed. Doing research about abnormally high oil pressure the only cause which makes any sense is that there is a blockage in a oil gallery or in the by-pass valve in the oil pump. Do you think that this was caused by the recent work on the engine. I have been quoted by another dealer that it would be $1100.00 to take the engine apart to determine what the problem is which is ridiculious since they determined the engine is shot. Car had no problems before the service. Any ideas of how I might determine what happened, or any test that you could recommend to determine what the problem is. I am fairly mechanically inclined and many years ago have changed engines out myself, but with cars today I prefer to let the experts handle this type of stuff. Thanks, Tom