I agree with most of your assessments, but I do not see this - and never have seen this - as a pure N + R = defeat of Mitt concept. I think a lot of Newt voters would support Mitt and I think a fair number of Santorum voters would also - if either were to go away. Not on FR or on talk radio maybe, but in the general public.
What we do know is this, there is only one state where the campaign was run in a manner that will defeat Obama: and that was SC. In every other state, Mitt has drummed down turn out with his negative assault machine.
That ain’t getting it done in November.
I can’t say for sure what drove South Carolina, but my opinion is that it was outrage over the media using Newt’s ex-wife against him. If I am correct, that’s a hard pattern to reproduce in other states, or in the general election.