I agree with @jochemvg. If we could make grenades stack a large part of the issue would be solved. Being able to decide in which order you draw your grenades already makes inventory management a lot easier as well. For instance, I always make sure my smokes are the last grenade I draw because I don't have to use them in a hurry, contrary to frags or flashes. If, in addition, we didn't have wait out the draw animation of each grenade and could just tap twice for our second grenade and thrice for our third grenade, the system would be fixed in my opinion.
In case the developers were to implement more hotkeys I would suggest using 3, 4 and 5 to select grenades in the order that you have them equipped, rather than having an array of hotkeys that is never used to its full extent (as mentioned in the original post).
I, however, would personally prefer keeping the grenade selection limited to a single key. This would translate better to console and, in my opinion, is more user-friendly. Furthermore there wouldn't have to be any adjustments made to the default binoculars and melee weapon hotkeys.