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: 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.
Question: Why did Sterling Holloway quit voicing Winnie The Pooh after providing his voice in this movie?
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.