When the Dr told Artemus West had until the 15th before it would be too late to give him the antidote for the plague, Artemus opened the calendar to the 17th.
When Jim shoots the anchor into the cliff face. The bullet would have had no place to go. This would cause the weapon to blow up. If by some miracle that did not happen the a bullet would be stuck in the barrel of the weapon. The night time he tried to fire it it would blow up. You can not block a barrel like that without an over pressure.
When the Caucasian messenger with the ransom note for Averi attacks West, he ends up falling into the ravine. The thing that falls into the ravine is an obvious dummy - dressed as a Native American man with long hair.
In the final scene, West and the girl burn all the money because it is infected. However, even though everyone there was infected, they don't burn anything else that they touched, such as their clothes or guns. Plus, the plague couldn't survive on the money for too long and it could have been simply quarantined (or decontaminated) until it was safe.