Nice write-up, I appreciate it. I installed mine today, and although it's not too hard to figure out, it's always nice to have a nice write-up with tips for install.
I took my bumper cover off, just makes this sort of thing so much easier. Plus I have a grill so I can't reach up through there anyway.
As to the orientation of the bracket, the rounded cut-out should be up (toward the sky), and on the passenger side of the subie emblem. It's there to allow the hood release latch to swing down so the hood can open. You can see exactly how that works if you have the bumper cover off. If you mounted it backwards I'd be worried that something's not quite right in there, you shouldn't be able to open the hood.
Did you notice on the diaphragms of horns (the black part), one is matte black and one is shiny black?
Edited to add: The youtube videos give you no idea how damn loud these horns are.