This, I think, is the starting point:

"If I eliminated beyond any and all doubt the possibility of ET, of secret govt craft or technology, intra-dimensional visitation, etc. what that put a dent in the number of sightings that pour in every year?"

If the answer is no then you have to weed out the hoaxers, the nutjobs and the misidentification of ordinary craft or phenomena. If there is the merest possibility of a sighting being attributed to any of these things, you get rid of it. What are then left with? There will be a tiny percentage that still can't be explained. And you have to start with those.

That's your starting point. Just those few cases would be a lifetime's worth of work.