The issue with investment cast NA recievers is not BS, its a legit concern. I have one sporterized that I hunted with for years. When I learned of the issues with them I contacted a friend of mine that does NDT and asked what he thought. He told me to bring it to him with the barrel removed. He magnafluxed it and ultrasonic tested it. He said there was a few "suspicious spots" in the back of the receiver but the threaded area and area behind the locking lugs was fine. If you are worried about yours then pay to have it checked out.
As far as someone stamping a 1 on a low number receiver to make a high number, I seriously doubt that is possible. Those 03 receivers are pretty damn hard, I think if you were to try you would crack or otherwise distort the receiver badly, not to mention having to match the numbering font exactly.