I don’t think there’s any need for a conspiracy theory here. Huckabee and Romney have weak support even among Republicans, and McCain appeals to the uncommitted centrists much better than the other two do; if the Republicans tried to eliminate McCain so that they could have a controllable lap dog of an orthodox Republican candidate, they’d be handing the election to the Democrats.
In 2000, when the opposition was Gore, eliminating McCain in favor of an orthodox lap dog was the smart thing to do because Bush was a reasonably credible candidate and could play the “compassionate conservative” card. In 2004, when the best the Democrats could come up with was Kerry, keeping the orthodox lap dog in office was again the smart thing to do. But if they eliminate McCain now, they won’t get an electable lap dog.