Question: Why would the loco derail if the siding switch was set to the siding where the freight train went?
Answer: It wouldn't derail, it would break the bolt on points - the part of switch that moves, causing the points.
Question: Why didn't they go back to the 3rd or 4th locomotive where they could access the brake hoses and disconnect one of those, instead of trying to get to the hoses between the 1st and 2nd engines?
Answer: That's what they were doing, going car to freight car break the hoses. They knew it would slow the train down but not stop it. Slow enough so they could jump off without getting injured. That's why they were desperate to get to the first engine not only to break the hose but disconnect from it.
Chosen answer: Most likely from the speed at which the train was going, a train is like driving any large vehicle. You have to maintain a certain pace, to stop at an instant, for the unexpected.