This is for a sit-down interview, where the camera is only
on the interviewee. Is it always preferable to use a
lav over a boom in this situation? When using a
boom, what is the mic-to-subject distance range that we should aim for?
And as far as positioning of the boomed mic, would about
45 degrees off parallel-to-the-floor, aimed at the mouth,
be a good plan?

It's generally easier to "get it right" with a lav, as opposed to a boom in these situations. But if you've got the right hyper, and the right boom op, then you can get a more "familiar" sound from the boom mic. Usually not so's the audience will really notice nor appreciate the difference in sound, but it's there.
Lav's are *usually* easier in these situations, and barring trouble with ruffling cloth, heavy nose breathers, long hair hitting the mic, or chest pounders....they're a safe way to go.