Please accept my rebuttal as friendly "sparring." That's the way I received your comments!
-CodeTRUCKER
Me too
When it comes to predicting a collision between planets, comets, etc, I do believe the math is extremely complicated. Predicting planet movements under the assumption for circular orbits (be they around the sun or earth) is one thing, but trying to predict the future path of a comet with potential to collide with the earth requires an awful lot of background knowledge together with extremely accurate observations..
So my suggestion for a disproof is that a)
If records of the Mayans math and science levels are sufficiently comprehensive, and b)
If they show that the Mayans did not have this necessary background knowledge -
Then one could say with significant confidence that the Mayan calendar ending is not based an an accurate prediction of an impending collision.
Two big
If's I know, but I just pointing out one way in which the Dark Planet theory could very genuinely be disproved.
As for why does the Mayan calendar have an end, I'd guess just some random religious notion, or perhaps the
prediction of a supposed mystic who was 'under the influence'.