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 did God choose Jerry to spread his word around to other people?
Answer: He tells Jerry he chose him specifically because he was an average person. As he put it "better than some, not as good as others."
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.