Fall as in the season, or fall as in overall fall of the culture? You're not really clear on that (nothing that a little grammar proof read couldn't help). If the Mayans couldn't even predict their own demise, I'm sure as heck not going to believe their calendar signifies the end of our days.
Almost all polytheistic religions and cultures have had at least one snake or serpent-like god/deity. The Mayans aren't really all that special in that aspect. Quetzalcoatl was the spirit of intelligence and the wind. Actually, you'll find in a lot of cultures that the serpent, or serpent deity, was synonymous with intelligence.