Interesting idea. After testing it a bit, I felt that 'tail-read' mode may be a tiny bit better if the bottom/active paragraph always started at the same screen

Interesting idea. After testing it a bit, I felt that 'tail-read' mode may be a tiny bit better if the bottom/active paragraph always started at the same screen location. That way the eye gets used to return to the same reading start instead of hunting each time. You already seem to be doing that that for single-para mode and it does feel more consistent.