Unfortunately, as you point out, using the Dashboard action in the main menu does leave the RB menu open when you use it. It's actually possible, I think, to build your own action that essentially does the same thing - toggle Dashboard - as well as close the RB menu. (I'm guessing that all the built-in Dashboard command does is really just to simulate pressing F12.) The only problem remaining is that this method won't change the active behavior, so you have to go back into the RB menu to close the Dashboard.

If it were just about me, I wouldn't even worry about it or look any further. But I pretty much want the thing to be so easy my mother could use it, so it would be great to have something even more intuitive.