Pronunciation of the possessive form of singular nouns ending in “s”

First, let me point out that this is a non-native English learner asking this question.

I know similar questions have been asked here (some of them found on this page), but (at least of those I saw) none exactly answer my question. If they do, point me to the right one (but please read the following first).

Almost always, I’ve added “’s” to singular nouns to form their possessive forms, even if they already ended in –s. And that’s exactly how I pronounced them: with an (additional) –iz sound at the end. Exceptions for this would include proper nouns like “The United States,” “Massachusetts,” etc. So I’d probably write and say “The United States’ president” rather than “The United States’s president.”

I’m not sure what I’d do with names like “Socrates,” “Archimedes,” or “Hedges.” But I used to think I had to add an “’s.”

Only recently I saw written forms like “Lucas’,” “James’,” etc.

Having done some research, now I know that how you would write these possessive forms depends on what style you use: the Chicago Manual of Style (CMOS), the AP style, or some other style. But still I’m not sure what this implies for pronunciation.

My primary question is: Does one say/read a possessive form exactly as they write it?

In his answer to this question tchrist believes this is (or should be) the case:

The most useful rule — and the most general and the easiest to remember — is simply that you add ’s whenever you actually say an extra /əz/ at the end when forming the possessive, compared with how you say the non-possessive version. Let your own ear be your guide. That’s all there is to it. No fancy rules full of exceptions. Just your own ear (as a native speaker, mind you).

But they also note:

Note that things like Ross’ and Chaz’ are always wrong, because no one says those with only a single syllable. That is a common error.

But one of the cases tchrist calls an “error,” i.e., “Ross’,” is exactly what the AP style recommends, and, correct me if I’m wrong, you can’t call a certain style “wrong.” One can only say they prefer other styles.

This seems to imply that you can’t count on a style like the AP style to tell you how to pronounce possessive forms.

But what about other styles? According to the answer Sven Yargs gives here, we read in Chicago’s Manual of Style (second emphasis by myself):

7.21 An alternative practice for words ending in "s." Some writers and publishers prefer the system, formerly more common, of simply omitting the possessive s on all words ending in s—hence "Dylan Thomas’ poetry," "Etta James’ singing," and "that business’ main concern." Though easy to apply and economical, such usage disregards pronunciation and is therefore not recommended by Chicago.

This seems to show that Chicago’s Manual of Style can be trusted to give us the correct pronunciations, but then, as Sven Yargs also notes, the same style “disregards” pronunciation somewhere else (second emphasis by myself):

7.18 Possessive of names like "Euripides." In a departure from earlier practice Chicago no longer recommends the traditional exception for proper classical names of two or more syllables that end in an eez sound. Such names form the possessive in the usual way (though when these forms are spoken, the additional s is generally not pronounced). [Examples:] Euripides’s tragedies[;] the Ganges’s source[;] Xerxes’s armies[.]

And looking for answers outside the circle of formal styles, it still sounds like we can’t trust the written forms to give us the correct pronunciations. I was watching Stranger Things with English subtitles, and while Mike said “Lucas’s” (with the extra s), the subtitles read “Lucas’.” Interestingly, the same thing happened with the word “princess,” while even the AP style doesn’t advise “princess’.”

Also, based on the answer by grandtout here, the written and spoken forms may differ.

So all in all, it sounds like while we can refer to a style guide to see how we should write possessive forms, we can’t necessarily count on them for the pronunciations. So what should be our source?

Of course you may say that we should see what natives say, but then that’s why I’m here: Please, tell me:

1. I know some people write, for example, “Ross’” (without an extra s), but does anyone say it that way? What about names like “James,” “Jones” and “Charles”? Because at least one user of EL&U, i.e., Natalie, says they pronounce “James’” without an extra -iz sound.

2. Most sources seem to suggest that longer names ending in s, especially those ending in an –eez sound, are usually pronounced (as well as written) without an extra s (See the 7.18 rule above).

Also, tchrist says here (emphasis by myself):

So words ending in unstressed /iːz/ are exempt, like for example this series’ end, that species’ demise, Mercedes’, Ramses’, Sophocles’, Socrates’, Achilles’, Diomedes’, Archimedes’, Eratosthenes’, Ulysses’. (But not trapeze’s, because that one is stressed! See how that works?)

Which of these rules do you agree with? What role does the stress play?

I’m sort of used to adding ’s all the time (except to plural nouns ending in s), so do I have to change my habit, or is it alright? In other words, would it sound too weird to the ears of a native If I kept pronouncing these longer names with an extra –iz sound?

3. Just as you see in the quote above, tchrist believes we should say “the series’ end” and “the species’ demise” without extra –iz sounds. But this is contrary to what I thought. In fact, they sound really wrong to my ears. Am I wrong? Do I have to change how I pronounce them? Considering that tchrist is right, do I have to pronounce it the way they say, or is it just a suggestion?

4. In many sources, I’ve read that one should add only an apostrophe to biblical/classical names such as “Jesus” and “Moses.” For example, here, according to Sven Yargs, we read in Garner’s Modern American Usage, second edition (2003) (emphasis by myself):

Biblical and Classical names that end with a /zəs/ or /eez/ sound take only the apostrophe: Aristophanes’ plays[;] Jesus’ suffering[;] Moses’ discovery[;] Xerxes’ writings[.] No extra syllable is added in sounding the possessive form.

But in his answer to the same question, tchrist says that nowadays people usually say “Jesus’s” and “Moses’s” (with an extra -iz sound).

What do you think about this?

And more importantly if, for example, the Jesus we’re talking about is some other Jesus, then what?

5. So far I’ve been talking about proper nouns. What about other nouns, such as the “princess” mentioned above, or the “class” here? Can they ever be pronounced without the extra s?

6. Does this rule from AP style reflect pronunciation? (It wouldn’t for me.)

FOR AP STYLE: if the word following the singular common noun ending in s begins with s, add an apostrophe only. (This includes words with s and sh sounds.)

One example would be “boss’ sister.”

7. As I said above, I prefer “The United States’” to “The United States’s,” “Massachusetts’” to “Massachusetts’s,” etc. But am I right in this case? And does anyone say otherwise?

8. Is there a source you can give for pronunciation of possessive forms? Can I trust Chicago’s Manual of Style everywhere except in 7.18 above?

9. Is there any difference between American and British English in the case of the questions above? I speak American English myself, but I would like to know about the British pronunciation too.

Thank you in advance.