McGrath -

'"Spell it the way you pronounce it" seems pretty simple and straightforward ... '

"Seems", perhaps - the problem I have with it is that in the normal run of my conversation, the possessive forms of Jesus, Mars, Bridges, Demosthenes, Cassius, etc., don't come up often enough for me to have unequivocal pronunciations of them. Whether in speaking or writing, I'm liable to make a quick mental query as to "correctness", and probably will moreso after this discussion; it is in that sense that I find the 1999 Fowler unhelpful.

It is unclear in your post, and I don't have a copy here of the "real" Fowler to check, if Fowler was talking about adding only the apostrophe after s in instances where you actually pronounce what the 1999 Fowler calls an "extra" s, as you imply, or whether he wasn't rather assuming that an extra s is not, in fact, pronounced. I'm not talking about his recommendation, but his description of the "formerly customary" practice. Clarification?