Your post contains a nice compilation of information (too bad you cannot post the links, - but that's an anti-spam measure).
I cannot speak for Kostya, Aquamail developer, and he will probably respond later.
But I can offer my view on some of the issues that I know.
AFAIK, Aquamail does use one of the mechanisms of autodiscover with EWS accounts (you can find a recent thread here discussing that).
I am not sure how widely Mozilla's based solution is spread out, on the server side. (Obviously, it is not that widely used on the client side: as you mentioned, none of the mobile clients use it.)
As for DNS-based SRV, - it looks like it hasn't picked up. Unless I am not doing it correctly, - I don't see, say, Gmail or Yahoo having SRV listings.
Moreover, if you look at the mozilla documentation page,
https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration , you can see that Thunderbird doesn't seem to use SRV.
But it does use the implicit convention used by many providers (even though Mozilla calls it "guessing"): smtp.domain.com, imap.domain.com, etc.
AFAIK, Aquamail is using the same as the default attempt of "guessing" for any providers that it doesn't know.
So, your suggestion sounds interesting, - assuming that significant enough number of e-mail providers follow those methods that you've listed.
(And I see it working both ways: if significant number of clients would support those, providers would start using them as well.)