Padme should die in Episode III. She is too important of a character not to. Though I don't want to see her die, her death would really add to the movie. And I don't think that Anakin should kill her. He risked getting expelled from the Jedi Order for her and it him great effot just to win her over. Why would he kill her? He loves her too much.
Anakin shouldn't know about the pregnancy either. Actually, I think that Padme should already have given birth in Episode III. She should try to tell Anakin but fears that the news might be a distraction for him. In the first place their entire relationship and marrigae is completely forbidden and having children just makes their situation even more complicated. Secondly, she knows that Anakin has a lot to deal with, with him fighting in the Clone Wars and everything.
In Episode III, I also think that GL should establish the idea that both Anakin and Padme are still deeply in love with each other, but that their relationship is now underlined with tension between them. Ultimately, I think that Padme should be forced to choose between Anakin and the Republic. She sees the darkness in Anakin and fearing for him goes to Obi-wan for help. Anakin finds out, and feeling betrayed blames Obi-wan.
I really like what you said about her having to choose. And, although I adore Padme's character and would hate for her to die...I think you're right. She is too great a character to be left in lingo.
Also, having it that the birth has already happened, or, happens at the beginning of the film, is a great idea. Another thing, ((I seem to like a lot of your ideas FJ)) showing how deeply the two of them love each other but also how intense their relationship is, or has become, is crucial.
this is what I will say about padme's death. George Lucas said that Anakin was a tragic hero. the literary definition of which is someone who starts at his peak and then tragically false to his lowest, hurting and alienating many he loves. look at macbeth and hamlet. now consider that and the news of padme's demise. Their's only one person that can and will kill her