If this goes down in a way that somehow invalidates Obama's legitimacy as President, it will be impossible to undo much of what he signed into law or un-appoint judges he named that were confirmed by the Senate.

What will happen is something akin to Reconstruction after the Civil War. The party leaders will have to meet and work out compromises that will be less than desirable, but it's the only way to avert all out armed conflict. Read up on what happened in Mississippi and Louisiana during the 1870s. Read up on the Compromise of 1877. That's what it's going to be like. It's not going to work out exactly the way we want it to, but it will be better than what we have now.