I have not read the novelization, but I have both seen the movie and read the original novella many times.
- Why did someone stick his hands into his mouth during this scene?
He didn't just stick his hands into the guys mouth, he was making sure the air passage was clear while the doctor performed CPR. Once he was sure the passage was clear, he grabbed the defibrillator. They had no way of knowing what would happen. The point of this scene was to be a surprise. This scene was not in the novella.
- Why did MacReady hold the dish near his face?
He didn't. It's called a camera angle. It was done at that angle for dramatic tension. In reality he was holding it out from his body. You'll also note that nothing happened previously so he didn't know exactly what would happen. In the original novella, it describes the blood flinching away from the hot wire in a test tube. MacReady likely expected something along these lines, not the sudden transformation we witness.
- Why would they allow themselves to be tied up?
Because MacReady was clearly ready to kill anyone who was unwilling to do so. It was a case of certain versus uncertain death. In the original novella, nobody was tied up because as soon as someone was identified as alien, the others ganged up and killed him immediately. But that was because more people were still left by that point.
I think you need to appreciate that the characters don't necessarily all have the same information that you have as the viewer.