Keyboard shortcut to allow opening unsigned pkg file

What works for me (although some quirks remain) is this: in view options (CMD-J) set up your finder window exactly how you would like them all to be. Make sure to check "Always open in ... view" and "Browse in ... view". When you're happy, click "use as defaults". Then open a terminal window and enter the following command:

sudo find / -name .DS_Store -delete; killall Finder

(it prompts for password and generates some "no permission" errors. Could take a minute or two)

It removes all the .DS_Store files from your folders. These are hidden files that contain the view settings of a specific folder (you might have noticed them when opening a mac USB stick on a windows machine or whatever).

It's not perfect, right size is relative to your finder window size i believe, but it's much better than the frustrating craziness that comes from every folder having different settings...

CMD-J opens the view options. When in that window, Fn will change the "Use as Defaults" button to "Restore to defaults". That's pretty close to a shortcut, I'll do some digging to find out how to make it full circle...

I think the OS X Human Interface Guidelines are the place you're looking for. They state that there are Apple-reserved shortcuts (e.g. cmd ⌘+space) and also “expected behaviours” (e.g. cmd ⌘+c) one should respect.

While they don't explicitly state the priority, it suggests itself that the System always should have priority before the frontmost (active) and then background applications.

From my experience, you’re on the safe side with multiple modifier keys (i.e. cmd ⌘+ctrl+shift+…).

As an aside: regarding your specific example (assigning a shortcut to open a new terminal) there's also excellent apps for that, TotalTerminal (Terminal.app extension) and iTerm 2 (Terminal.app replacement) come to mind