ABC Extended Keystrokes- Why do they only work for some, but not all?

I extensively use the ABC Extended keyboard to type diacritical marks for cataloguing Middle Eastern and South Asian manuscripts and also writing my dissertation. It's strange, but certain commands works with some letter but not others. For example, I can type ⌥x then s to get ṣ, but I can't type ⌥h then s to get s with a macron underneath, s like I can ẕ (note the slight difference in the underline resulting from the keystroke versus command u to underline a single letter, which is a pain in the butt... See this link—albeit in French— explaining that uppercase s with submacron is U+0053 with U+0331 and lowercase is U+0073 with U+0331). We use this character to render the Perso-Arabic script letter ث under the American Library Association and Library of Congress (ALA-LC) romanization standards used in most libraries for both Persian, Pashto, Kashmiri, Ottoman Turkish, and Urdu materials, among others).

Oddly enough, when I look in the accented characters list in the Character Viewer, I don't see an underlined s, but I might be missing something? I can type ş š ṩ and even (!) but not s with a sub macron.

Another feature that would be really nice to see added are letters with diareses above and below. Currently ⌥ u works to generate many letters with umlauts, but I only see one u with diaresis in the Character Viewer. We also use the letter Z̤, z with diaresis below to render ض in ALA-LC romanization (read more here, uppercase: U+005A with U+0324; lowercase: U+007A with U+0324).

Finally, it would be nice to see a Perso-Arabic script keyboard for the desktop that combines the extended character set used throughout South, Central, and West Asia, from Persian in Afghanistan to Uzbek and Urdu—even Ottoman Turkish— much like you can now on the iPhone. Currently, I have to switch between several keyboards and the Character Viewer to do what I need. Something like a "Arab-Extended" version, analgous to ABC Extended?

Long story short: With everyone working from home it would be nice to see Apple make it just a little easier for us to do it. Your keyboards have progressed little since the early 2000s! Pay attention to those diacritical tables on the LC website posted above, and others used in the EU and elsewhere. You will see them used in major resources like WorldCat. Doing so would allow the library, archive, and museum communities across the globe to more speedily increase discoverability and access for these materials.