Just for reference, but you typically show signal flow from source out (i.e. from computer to interface to speaker). In other words: PC/USB -> L/R Mbox 3 Mini -> 1/4" to XLR to Mackies.
With regard to being able to A/B monitors, I'd suggest a monitor manager/switcher. There are a couple in your price range (Behringer & Samson) that should adequate meet your needs and give you some extra features you may or may not need at some point.
Also, don't forget the NS10s will need a power amp to drive them since they're not internal amplified (powered). Given the amount of money you're looking at spending, it makes me wonder if something like the Focusrite VRM box might serve you well. It allows you to test your mix on a variety of monitors through headphones (by virtual modeling). Just a thought.
^ What he said, but I would shoot for a pretty good monitor control, because it will effect the sound of both speakers running threw it. If you get a ****ty one, it could end up making both your speakers sound not so appealing.