I’ve seen these two approaches for /hcircumflex and /kcaron: 1) To put the diacritic at the top of the stem (as in Alegreya), or 2) Floating somewhere near to the visual center of the letter (as in Georgia or Palatino). And even mixed solutions, as in Athelas. I am not sure which one is better. What are your thoughts on this?
Nested marks seem problematic as weight increases.
I think the nested marks generally look noisy, and are less clear than having the marks above the ascender height.
That blank area is part of the letter h, part of the pattern we recognise as h (that’s why I dislike nested diacritics sitting in that space). I take the view that the letter should be stable enough to carry the optically centred mark above, and particularly in smaller text sizes this positioning is helpful to reading. I can imagine a different solution for larger, display lettering.