I worked for a major automotive component supplier who designs and builds parts for most of the major automakers. I wanted to make a few of relevant observations from my experience.
First, all parts were extensively tested for function and safety. Designing a good test that is representative of years of field use is very difficult, but none of the automakers seemed lax in their testing requirements. Some were pretty quick to dump performance for a cost savings, but I don't know any who were, these days, willing to sacrifice on reliability. There weren't many arguments with customers about the cost of testing, and it was generally thought that some tests demanded by OEMs were needless, but we'd gladly take their money anyway.
Second, parts were regularly improved based on analysis of returned parts. The best source of these were fleet vehicles, which provided lots of high mileage parts back to the OEM--each and every one of these returns was examined, graded (often by some poor intern), then archived for future reference should a problem develop. I remember one incident where some tiny steel spring clip broke--this had never been seen before, so the entire engineering department was re-directed to determine the cause. Thousands of old parts were pulled out of storage and re-examined. I don't think we found another broken clip, but it was a big deal.
Lastly, parts were frequently revised for better performance, lower cost, or better reliability. Little bits and bobs, like switches, valves, fasteners, connectors, etc., were often used on numerous vehicles by a number of manufacturers. Each part had at least two sets of drawings and part numbers. One set was for our use, as the supplier, and had every detail labeled. Another drawing was prepared for the automaker, with only the details relevant to them called out explicitly. It was, in a sense, an engineering contract--we'd agreed to provide everything as described on that drawing as the same part number, but were free to change things not called out. Once I pulled up about thirty drawings produced for the same part, a tiny thing used in many of our products, to see whether we could change the part to an improved steel that was cheaper and tougher for this application. In all of the automaker drawings, the material spec was loose enough for us to change without asking for a change in the drawing. Our internal part number did change, but as far as they automakers were concerned, they were still using the same part.
Anyway, it's quite possible that someone might make a fix to the ignition switch without GM even knowing, and certainly without requiring a change in part number. In my experience, all of the majors are actually pretty good about testing everything and they all really do want to sell people reliable cars, as even the US big three have come to realize that each lemon they put out there can sour a family of customers on their cars for life. Management can be boneheaded about a lot of things, but I really don't think this is one of them. 100% safety isn't possible, no matter how much is spent--but they all get pretty close. Just look at how the fatality rate has plummeted over the last few decades, despite more traffic and more collisions.