"The popular national chain says they have no reason to believe their error was the cause of my misfortune. It must be a "coincidence" and that they have no knowledge that such a catrostrophic event could be caused by overfilling the oil. (They do admit to overfilling the engine with oil.) "
They are either idiots or just playing dumb & hoping you will go away. Tester is right. With the proper amount of oil in the engine, & with the engine running, the level of the oil in the oil pan is below the spinning crankshaft. Too much oil will cause the scenario Tester described.
Don't get rid of the oil! Go to another garage & have them drain & save every bit of it. Or maybe do this yourself if you feel comfortable with this. Plus pour out & save what's in the oil filter. My guess is it was at least 3 qts overfilled.
Call the local vo-tech school & ask to talk to an auto instructor. Now he's not gonna go to bat & testify that the garage ruined your engine, but what he could do is sign a sworn affidavit stating that if an engine is overfilled with oil it will eventually be ruined.
Then file a lawsuit (if it comes to that)& with the affidavit & the fact that the garage admitted they overfilled the crankcase, coupled with the fact that you don't have to prove beyond a reasonable doubt in a lawsuit, just to a "preponderance of the evidence", you would probably win.
GOOD LUCK & PLEASE POST BACK TELL US HOW THIS TURNS OUT!
Edit: Once at the Chrysler dealer I worked at, a young mechanic forgot to put oil in a car w/3000 miles on it--after he had drained the oil & changed the filter. The customer made it 4 miles before the engine seized. Now this is with NO oil.
With the engine grossly overfilled, the spinning crankshaft will whip the oil into a foam like Tester indicated. Since foam is 99% air, and since the oil pump is only designed to pump liquid, there of course will be no oil pressure at times, & when the oil that's not foamed up goes into the oil pump, you will have OK pressure.
This is why a grossly overfilled crankcase won't ruin the engine right away. Joekota said it was 175 miles and 5 days after the oil change that the engine failed.
In 1994 we had a guy come in the garage with a late 80's I think, T-Bird w a 351 CID engine that was making horrible mechanical noises from the engine. He had just had his oil changed at a fast lube place 4 days earlier. I found 10.5 quarts in the lubrication system as opposed to the proper 5 quarts. The engine was shot. This time frame from oil change to engine failure is in line with Joekota's scenario.