Miz and Maryse should absolutely, without a doubt win. Even if they didn't carry this feud (even Maryse, she did a pretty good promo adding another layer to the story), they should still win because both Cena and Nikki are expected to leave for a while. It's the smart move to put over the people that are still going to be there. Especially if it's The Miz who should be WWE Champion right now (maybe a tad bit of bias in that last sentence).
As for the Cena proposal thing, I don't like it. Not that I don't like the idea of them getting married, but if he does it, it proves The Miz right in everything he has said which will be terrible if Cena and Nikki win. He'd prove it because throughout all of Nikki and John's relationship, he has told her that he doesn't want to get married and he doesn't want to have kids. Nikki stays with him and expects him to change, but that's another conversation for another day. If he proposes at WrestleMania, he's just proving that all he cares about is his public image. He doesn't marry Nikki for all these years, but then suddenly they're put on TV together and he proposes at the biggest show of the year. Oh, how convenient.