Re 1: The parties are so polarized at this point that there really isn't anyone in either party who is close enough to the center that they'd be "out of step" with the party as a whole.
Responding to points 1 and 3, I think you have a point but I don't fully agree. Yes the parties are much more sorted, making party switching less of a natural ideological move. However, both parties still have centrist wings, and these centrist politicians aren't totally out-of-place in either party. For example, from an ideological perspective, I don't think Susan Collins is really that much more conservative than Mark Pryor. We've also seen party switchers move to the right/left after switching (e.g. Specter), and I would expect the next party-switcher to at least somewhat change their voting behavior.
Responding to point 2, I actually wasn't just talking about 2014, but the near future as well. Not that I would bet on it, but 2016 and 2020 could be realigning elections.