Now we typically field hunt so unfortunately I don't have a lot of experience on the water. Plus our ducks and season are early so we get all of the uneducated young ducks and that makes life pretty simple.
All of that aside, and speaking to adding a gadwall to the spread, I've done a lot of research and for the mainstay, guys are recommending adding all types of ducks and decoys to the spreads. They say that even just the different brands of decoys simulate ducks in different ages and stages of feather growth. When we hunt on the water here we see all kinds of ducks (Northern shovelers, mallards, blue and greenwings, buffel heads, pintails, red heads, canvasbacks, mergansers, and wigeons) so I wouldn't be afraid to throw any type of decoy out. The biggest thing to our spreads would be to have the motorized wing decoys. The effectiveness of these decoys have decreased since their début a few years ago, but they are still a major component of getting airborne birds to see the spread.
As for your mojo battery issue, our batteries don't last much longer than that. Only fix for that is to bring more batteries out or rig up a remote for the duck to shut it off when there are no ducks around.