Ask yourself this. If that many people spotted it and reported it, why didn’t they find the shooter?
Occams Razor
Maybe good planning? Maybe he didn’t fire in a crowd full of people and had plenty of time to GTFO?
Maybe htey did find the shooter. The real question is why didn’t they tell us who the shooter was.
Actually the simplest explanation IS a missile. The most unlikely is that of a wire sparking in the fuel tank.
Occams Razor
You're exactly right. And that's why we know, for a fact, that "they" DID find the shooter.
Now, why do you think they'd tell US that they found the shooter?
Because they did not WANT to find the shooter.
Look, you seem committed to your reasoning. I really am not in the convincing business. If 200+ eye witnesses mean nothing to you, so be it.
The FBI took over the crash investigation from the NTSB, and despite interviewing a whole slew of people, managed to avoid interviewing the people who had seen what appeared to be a missile.
You don't find what you don't look for, especially things you refuse to see.
Traces of PETN on seat fabric were blamed on using the aircraft to train explosive sniffing dogs, but iirc, the people who acquired the fabric and had it tested were arrested and prosecuted for interfering with the investigation.