There's no chance Obama loses outside of some insane freak event. People who think otherwise are delusional.
would it be theoretically/formally possible for both parties to nominate the same man/woman?
Yes.
Well, at least I know it's allowed (or at least was) by the Republican Party's rules. I think there have been a number of state Democratic Party nominations of Republicans as well, but I don't know if the national party has an official position. In either case, there are instances where Governors or things like that have been joint nominated against Independents. (Usually strong independents who are likely to easily win a three-way race.)
Although, it'd be easier to win the Republican primaries as a crossover than the Democratic ones since the Republicans have winner-take-all. You can ask some Freepers about this and Johm McCain.
Regarding the state ballot question, most will allow double listing and then combine the votes. I currently can't think of an example to cite but I know there have been instances outside of NY's weirdness where they took multiple ballot listings + write-ins and added them together.
Won't start on Metcalf.