15 years later is a long time to mix up the sequence of events. That could happen to anyone. That is a more reasonable excuse than JFKs body was switched.
Actually, that would have to be the collective memory of a substantial number of people that need to be mixed up about the same sequence of events.
Reasonable of not, real life is often stranger than fiction. Just because you can't wrap your head around a particular scenario doesn't mean it couldn't have happened or didn't happen.
You have to take what the individual witnesses have said at face value and then look at the overall picture. When you do, you will find that those witnesses support eachother to such an extend that it would be nearly impossible for the sequence of events being different.