In some faces, the center strokes of the capital M touch down to the baseline, while others float above, either at the X height or some other position. Is there a term for this?

I have never heard a term for it. And I doubt it exists. Mainly because that’s just one of hundreds of type design decisions with gradual options. We might clearly separate structural differences, like a one-story and two-story “a” or the difference between a rounded or straight letter part, but that M section can end really anywhere, so I think it’s unlikely that certain options (from an endless amount of gradual options) or the design choice itself gets its own term. I would probably call it “high/low vertex of the M” while describing a typeface.