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: How is Elliott spelt, as I've seen it both Elliot and Elliott on various different Disney memorabilia?
Answer: In the original film, the credits show his name as "Elliott" (with 2 t's). In the remake, his name is "Elliot" (with 1 t). So older memorabilia will have "Elliott". In the original film, Elliott was the dragon's given name, in a manner of speaking. In the remake, Pete names the dragon after the puppy from his favorite book which was called "Elliot Gets Lost". This is the only reason I can find for the spelling change.
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.
Question: How does the Liparus explode and sink at the end? After the nuclear subs have been destroyed, the ship just starts blowing up for no apparent reason. Was there any explanation for it?
Chosen answer: When the nuclear subs have been destroyed, there's an explosion in the control room, and one of the men cries out "Fuel tank!" Liparus has numerous internal fires raging after the battle with her crew. These uncontrolled fires eventually spread to her fuel tanks and ammo storages, one of which explodes in a huge fireball and finally causes Liparus to sink.
I thought the final explosions that sank the Liparus were deliberate self destruction after completing the mission in an attempt to destroy any evidence, rather like when Bolfeld manually triggers explosives after being thwarted in You Only Live Twice.
Liparus sinks slowly, and that would leave her crew plenty of time to evacuate. Angry and armed crew with a grudge against Stromberg.
Question: When Brutus and Nero capture Penny trying to run away, Brutus holds her by the straps on her dress. but when returning to the riverboat, Brutus holds Penny upside by her underwear. If it were possible, how would Brutus have managed to turn Penny upside down like that without releasing her and letting her run off again?
Chosen answer: It could be she got loose again when they were heading back and Brutus grabbed her again but this time by her underwear.
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.