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

Solution 1:

Here's my tuppence worth as a native British English speaker:

1. The majority of nouns ending with one or more Ss have their pronunciation adjusted when attributing possession by simply adding the sound -ez (or -iz, depending on your accent). This is entirely independent of how you might write it (thus addressing your primary question).
2. As with anything in English, there are exceptions. You will learn these exceptions as you go.

On the exceptions front, it can be that you would actually restructure your sentence to avoid saying something that sounds silly. For example, rather than say (note that I am showing pronunciation, not spelling) "the United States-ez foreign policy is currently isolationist", where saying "United States-ez" may sound wrong to you or the listener, you might instead say "the foreign policy of the United States is currently isolationist". Another example might be "Socrates-ez teachings really make you think" which would more likely be said as "The teachings of Socrates really make you think".

For the names Ross and Chaz, you would always say Ross-ez, Chaz-ez. As I said, you do this irrespective of the way it would be written. You can even apply the same to any non-English proper nouns, like Jesus-ez or Ramses-ez, but again just from the pronunciation perspective.

In summary, I entirely agree with your quote from tchrist:

Let your own ear be your guide. That’s all there is to it.

So for the non-native speaker, add the -ez and be assured that you will be understood. Listen out for a native speaker avoiding "silly sounding cases" and adopt those yourself.