The mixing method isn't so much a matter of taste, it is a matter of
making a yeasted bread that needs excellent gluten formation in order
to support a HUGE amount of fat and "other" stuff. Fats prevent
gluten formation -- small amounts, no big deal, but 9 ounces is a LOT
in a loaf. You can make the brioche the way you otherwise would, but
the amount of fat and eggs mixed in up front will prevent an adequate
amount of gluten formation, and you will end up with a rather squat,
cake-like loaf (at best) rather than bread. I don't have that book
in particular, but it should also be putting particular emphasis on
keeping the dough cool (well below the melting point of butter) for
the incorporation of the butter, or you will have pools of
butter. The eggs go in before the butter in order for the lecithin
to work it's emulsifying magic when the butter arrives. I would have
hoped at least some of that is in the book, but if not, may I
recommend Shirley Corriher's "Cookwise" for an excellent, readable
discourse on this very subject.
As for the 500F oven, have you moved your thermometer around? Are
you sure you have no significant hot or cold spots in your oven? I'd
say an hour is a bit overkill, but a long preheat helps even hot and
cold spots in most ovens by insuring all the firebrick and metal are
at operating temp.
HTH
Joe