“This accident will have been preventable”.

The question seems open to interpretation (which is bad for surveys like this).

If I visit a location that was the site of a mass shooting a few years later, have I been “physically present on the scene of a mass shooting”?

I think you could reasonably answer yes: you’ve been to the physical place where it happened, even if not at the time it happened.

Where on earth do they get the ‘AYD’ from?

Seems like spifk, sporf, or knorkoon might be more sensible.