That Walmart battery is the exact same one I bought, and that CCA rating is higher than BMW's battery has, so it's not the problem.
If your new battery discharged overnight after one day in the car, your problem is that you either have a serious parasitic drain in your car, or you bought a dud battery. Did you measure the battery voltage before installation? With extra electronics in your car, you really need to do the test for parasitic drain and eliminate all possibilities.
The Everstart MAXX H8 doesn't come with a vent tube (you use the original BMW vent tube with the plastic elbow from the original battery). It has two vent ports on either end, in the same exact locations as the BMW battery. It is essential that the battery be vented with the tube, unless you want corrosive/explosive gases in the trunk. The red plastic postive terminal cover on the Everstart has (or should have) a small plug attached to it that you remove and insert into the vent port that isn't used.