Thanks for looking into this — I agree with your reasoning. In fact, I implicitly assumed that the current implementation uses the scheme you suggested because it makes the most sense to me.
I would propose a more radical UI change: if f is the luminance mapping, the current UI makes the user set f(x) - x, so f' < 0 or f' \approx 0 can happen very easily without intention.
The user should set just f, extrapolated with slope 1 at the edges. This is how most tools do it (eg Photoshop, Lightroom, although in finite display-referred domains).
Also, the interpolation can give wacky oscillations around control points. Cf