I don't think they've expressed a position publicly, but a number of guides do contain discussion of setting-specific material -- i.e., "this works particularly great if you're a mystery cultist of Desna", etc.
Yeah that's cool. I doubt his would really step on it badly.
I would agree that it's a good idea to highlight 3PP content, because a lot of people are fussy about that -- some because 3PP can't be used in organized play, others because they just want a bright line around what's canon and what isn't.
Yup. My group in particular has a really fond appreciation of the work that Paizo has put in making sure that the vast bulk of options are, well, "Different, but not necessarily better." We don't allow 3PP material at all because of what we all observed with 3.0 and 3.5, where publishers determined the fastest way to sell books was to just make them more powerful!
A guide that includes 3PP and doesn't go to extra lengths to point it out is really of limited usefulness to me personally, but YMMV.