How can I set the default browser depending on time of day?
Not nearly as complicated, but perhaps of value, is the app Choosy. It allows you to choose your browser every time you open a link outside of a browser. While this might sound onerous, it's actually pretty transparent, and I find it very useful in the leadup to going live with a new web project and testing in multiple browsers over and over and over…

Might suit the bill if one added keystroke isn't a problem. Of course, once you're inside Safari, links clicked will open in Safari (although there's even a way to modify this behavior.)

FWIW.

Choosy