Oh, that's so interesting! As rachelindeed said above, it does seem like a lot of the things that I think of as British/American usage differences (e.g., what you mentioned above about that distinction of using "" only for reported speech,= and '' for any other kind of quoting, or the whole Mrs./Mrs question) are fairly recent phenomena, and that as recently as a few decades ago, our usage had *not* yet diverged. Fascinating!!

It's also so interesting to me what you said about -s' vs -s's. Because when I was asking around about this a bit ago, the British people I asked varied on whether they thought both were okay or only -s's was okay, but they all mentioned some variation of what you said: that technically the two spellings represent the two different pronunciation possibilities. Which makes sense, to look at it that way, but it had NEVER occurred to me! (Can't speak for all Americans, but I do think that idea is at least somewhat less on our radar.) To me -s' and s's were always just two variant spellings of the exact same thing, grammar-wise, and not at all connected to pronunciation.

Language will never cease fascinating me!