I'd be interested to know the opinions of punctuationists.
You wish is my command ...
Firstly, it's extremely rare for proper nouns for anything religious to retain the apostrophe for the possessive form they originally were.
So you need to use the possessive form of "St Andrews", not of "St Andrew's".
Two styles exist for the possessive forms of singular or uncountable words ending wit 's'.
(A) One style, I guess the most commonly used style, is never add an additional 's' to form the possessive of any word ending in 's'. [But I'm not 100% certain I understand what that style specifies.]
(B) The other styles requires to consider what happens if you remove the 's' (sometimes 'es') from the word you need to add your apostrophe to creating its possessive form.
Note you look at that as a distinct word without regard to its meaning in a noun phrases, for example, what happens when you remove the 's' from the word 'states' used in the noun phrases 'United States'?
If removing the 's' produces the singular form of what is a plural word, then add another 's' after the apostrophe.
If removing the 's' does produce the singular form of the word you want, then add just the apostrophe.
* belonging to United States ... removing 's' from 'states' changes a valid plural word to its singular form, so just add the apostrophe, giving the "United States' ".
* belong to me ... removing one 's' from 'Ross' makes me sound like a girl, add an apostrophe and an 's', giving "Ross's"
Both styles have some advantages.
Style A allows you to be consistent without having to think too hard.
I recently decided to change from a lifelong preference for style A to style B. I had what seemed like a valid reason at that time. Trust me, I want to be VERY SPECIFIC when writing about anything that belongs to me. :-)
I cannot even think of an example now, the situation where is happens is very rare, but I concluded style B would sometimes allow a writer to distinguish between when they wanted to show the singular sense of an uncountable word ending in 's' from the plural sense.
I have not been satisfied with what sometimes happens when using my newly adopted style, and have sometimes rejected it.
Aesthetically I dislike both rules when it comes to people's names. At least where I come from, people will add an extra 's' when saying the possessive form of a name ending with an 's' sound, but not for names ending with a 'z' sound. For example, I have certainly heard people say, "Ross's monstrous ego," but those same people would say, "Prince Charles' monstrous ears (or wife)."
So my new personal practice for personal names ending in the letter 's' is to add an extra 's' when the name ends in an 's' sound, but not when it ends in an 'z' sound. Otherwise I add an extra 's' when it's required for the style B listed above.
To AJ, there are a variety of styles of may choose from, as always I suggest just be consistent. One commonly style would require an extra 's' after the apostrophe, but as that is not how people would pronounce it, I suggest you adopt a style that allows you to use "St Andrews' graveyard".
Thus endeth today's sermon from Ross's Gospels.