I used Boxer for a little while, but there's no way to do a quick melee while you're dual wielding. If you want to melee, you have to first drop your second weapon with the Y button and THEN you can hit the left trigger for a melee. Also, putting grenades on the B button is just as bad as putting melee on that button.
I've had excellent results with the Green Thumb setup. Zooming isn't something I do much (I suck at snipers and BRs anyway), and pressing B to zoom doesn't really hurt you any. You can still do a quick melee while dual wielding by jamming down on the right thumbstick, and you don't have to move your thumb (preventing you from aiming) during either grenade throws or melee attacks. It's by far my favorite setup.