Question: How was Obi-Wan going to become more powerful after he was dead?
Answer: I agree with Darius Angel's comment. I also think Vader expected a certain sense of satisfaction after defeating his former master. In reality, though, he was still "owned" by the Emperor and was living with the consequences of his choices. Defeating Obi-wan did not change much for him. Obi-wan, however, gained the benefits that Darius Angel mentioned.
Question: Okay, so the three astronauts are on the run but only James Brolin makes good his escape. What happened to the other two? They're shown as being caught, but what happened after? Anybody know?
Chosen answer: It's never stated in the film, however, as the astronauts have been declared officially dead and therefore cannot ever be allowed to contact anybody, they were most likely executed as soon as they were captured.
Answer: How were they able to fire their flares, then?
Answer: O.J. Simpson would have had time to shoot his flare to show he was caught. With Waterson harder to believe, he had climbed the side of the mountain and was just coming to the top with the planes coming into view. Either he would have been dropped and died on way down, not getting the chance to shoot the flare, or he was grabbed from the edge. Unlikely to have time also to shoot the flare before being intercepted.
It's likely he would have had time. He was no threat and not going anywhere, so the helicopter pilots would have been in no rush to grab him. They were just sat patiently waiting for him. By the time they'd got out and reached him, he'd have plenty of time to grab and release his flare.
Question: Why were the aliens abducting people and why did they bring them back?
Answer: It wasn't definitively answered, but it appears the aliens took people in order to learn more about humans. It's unclear if all those who were returned had originally gone willingly, but the intent was not to keep them indefinitely or harm them, and they were returned to Earth, albeit many decades later. At the end, after the aliens had made contact, a new group of humans, including Roy, went with them voluntarily.
Answer: It is heavily implied that one who becomes a Force-Ghost achieves an untold level of power upon entering the state. Given that they have become a pure entity of the Force, it seems to back up the statement. Obi-Wan also becomes free to assist Luke in any case.
Darius Angel
In addition to this answer, I think Obi-wan also became powerful because he "let go." He did not feel a need to defeat Vader on this occasion - he was willing to surrender the fight and "move on" to a new state. Something that Vader might not currently understand.