Answer
Bob:
I probably shouldn't be venturing into this messy situation, for several reasons: (1) usage changes over time; (2) if you have contacted other experts, you'll probably get a variety of answers; and, (3) there are exceptions to "rules," and, in this case, the rule of "what sounds best or looks best" should be used.
That having been said, I believe that "Hodges' Harbrace Handbook" [I've used the title page which has the apostrophe AFTER the "s"] cannot be surpassed. The latest edition, however, may differ from earlier editions, because this "possession problem" has been around for decades [centuries?], and it has been addressed in different ways over the years.
I have checked six other grammar books, including the two that I rank immediately after Hodges' work: "The Heath Handbook" and "The Holt Handbook." They are in agreement with Hodges.
Let's take your two examples in reverse order:
First, there is NO way that guys' cars and gals' boots can be other than what I just typed. I don't even want to hear the incorrect pronuncation of "guyses' cars."
Second, Jesus has always been a problem. [Interestingly, Jesus and his apostrophe USED to be mentioned in grammar books. Of the six books I consulted, he does not appear.] The closest I could come is from Prentice-Hall's "The College Writer's Reference" [second edition]:
The following is directly quoted from the book --
Rule 34A: Use an apostrophe and -s to form the possessive of a singular noun ending in -s. If pronouncing the additional syllable is awkward, you may use the apostrophe alone.
EXAMPLES:
Don't waste the class's time.
The company produced Yeats' cycle of plays about the Irish hero Cuchulain.
"The Holt Handbook" also covers this topic. Quoting directly --
RULE 29A:
With singular nouns ending in -s:
To form the possessive case of singular nouns that end in -s, add -'s in most cases.
EXAMPLES:
Reading Henry James's "The Ambassadors" [I cannot use italics at Allexperts.] was not Maris's idea of fun.
The class's time was changed to 8 a.m.
However, a few singular nouns that end with an s or z sound require ONLY an apostrophe in the possessive case. This is because pronouncing the possessive ending as a separate syllable would create an awkward-sounding phrase.
EXAMPLES:
For goodness' sake -- are we really required to read both Aristrophanes' [sic] "Lysistrata" and Thucydides' "History of the Peloponesian War"?
NOTE from me: It's unfortunate that Aristophanes' name was misspelled in a grammar book!
My final answers:
guys'
gals'
Jesus'
Not that your students will believe me!
Ted Nesbitt