You turned down a free brand new replacement?
I suspect S&W offered it to you because they were just going to pull the slide off it and figured they might as well make you happy and offer you the whole gun. After multiple critical parts failures, numerous trips to S&W and a bunch of free mags, S&W finally replaced my P22 and I am real happy with the "indestructable" version they sent me. But like others have said, the P22 is made by a company that makes airguns and now thay make the "Colt" .22 A.R., which I hear they are having problems with as well.
You said you have several S&W M&P 15's, any of them .22? S&W had major problems with the extractors blowing off and firing out of battery in the first production models, they handled that problem very quickly and efficiently and sent all rifles back with a free mag, just goes to show, guns can leave the factory with problems/weakpoints that the manufacturer is unaware of, but once they become aware its up to them to handle it in a way that keeps thier customers happy and doesnt damage the reputation of the company. Glad it worked out for you, if your still not happy, I have no doubt you would easily recoup your money and probably even make a little more than you paid.