Human beings constantly set up systems and then tinker with them. Sometime in 1200's, somebody finished up and major scale, the minor scales, and the modes. Two weeks later, somebody started tinkering with them.

If a song is accompanied mostly by chords with an 'm' in them, then for all practical purposes it's a minor song. It could be modal, but who cares, really?

There are hundreds of tunes that start out minor, go to major to create some interest, then go back to minor. The reverse is also true.

Speaking of tinkering, that song by Thomas Mudd looks to be in G/Em, but it includes several G#'s and D#'s.   It's a real original. Thanks for the link.

I have another song for the list: Drill, Ye Tarriers, Drill.