David Frum is a Canadian who comes across as a "conservative" because he's a big-government globalist who was an ardent supporter of a strong interventionist foreign policy under both of the previous two administrations.
His real problem is that he has a secularist Canadian view of government -- mainly in the form of his views on three issues: (1) he supports taxpayer-funded health insurance, (2) he's a strident leftist on many "social issues," and (3) he has no respect for the role of constitutional law in the U.S. (particularly when it comes to the right of free citizens to keep and bear arms).
These gyrations we see and hear from him in recent weeks are the result of his pathetic attempts to reconcile his secular Canadian "conservative" values with the American political system. It simply ain't going to work.
I think you've nailed it. Canadian conservatism doesn't directly correspond to American conservatism. He passed as conservative because of his positions on foreign policy. But he doesn't seem to understand the philosophical underpinnings of American conservatism. The whole "limited government" thing probably doesn't mean much to him.
And he misunderstands conservative views on race. The Democratic Party is the party of the race obsessed; Limbaugh has criticized them on that quite frequently, their inability to see past race, their inability to conceive of a color-blind world, their inability to judge a man's character and philosophy without taking notice of his color. They can't do it, they can't believe anyone else can do it, and they don't believe its necessarily a good thing if you could do it, which they don't believe you can.
Thats Limbaugh's criticism of Democrats, and Frum sees that as racism, because he comes at race from the same angle as any Democrat. The absense of racism, the rejection of race as an indicator of character and philosophy, he interprets as racism.
So, no, he is no Republican, and he is no conservative in the American mold. He doesn't get it, and he doesn't get what he doesn't get.