Question: Okay, so I can't figure out why Paul and Matteo had the whole elaborate ruse set up. Why not just come out by themselves? Why did they set up a stranded boat and take the crew hostage with all of the events in the movie?
Question: Is there any truth that an anaconda will regurgitate its prey so it can keep feeding, or was it made up for the film?
Answer: A snake can regurgitate its food, but they do this mostly if they are in danger or stressed, not to go after another prey or swallow it again so it's easier to digest. The food is usually quite a weight on their body and takes long to digest, so the snake could regurgitate it to become mobile again if threatened.
Question: What happened to the man who shot himself at the beginning of the movie? Was his body devoured by titular anaconda?
Answer: The reason he shot himself was because he was being chased by the anaconda. Since the anaconda doesn't care if its prey is alive or dead, yes, he was devoured.
Answer: They didn't set it up. They were chasing after the anaconda anyway and were stranded. They still wanted to go after the anaconda after they were picked up, so they lied to the crew. They knew the crew wouldn't go after the anaconda on their own.