uh...let's go one question at a time shall we?
in ten words or less, it was all a dream.
in more detail, some kind of almost-all-powerful being created a sort of future (much like what the ellimist did in #7), but in sort of dream format, meaning it was all happening inside Jake's head, so that he could, well, learn about Jake and humans. The morphing problem is sort of the powerful being, messing with Jake's head.
No one knows which choice he made. He could've chose cassie, he might've chose the radio (I think it was a radio), some say he found a third choice.
No one knows who the voice belongs to either. Some say it's the ellimist, some say crayak, some say the one at the end of the series, some say it's the being who banished crayak (mentioned in #26). as for me, I think he's a new character altogether, with no mention of him before or after.