Yes both are separate.
Well the reservoir-valve is closed when the cell is turned on so it's not different as if the reservoir-hose wasn't there or on the other side. The one thing i'd change is drill another hole at the top so that the compensation of water/gas inside the cell is easier.
You don't have figures about your NaOH concentration in your cell, do you? I'm suspecting that if one used 25% electrolyte like commercial electrolysers the MMW would be much higher, only dependant on the conductivity of the water.
There is a video in which they test the gas output of the Anton cell, with the same amount of KoH concentration (3%) it gets more or less exactly the same MMW value, much like many other cells they tested. (4,7-5,0 MMW)
I then doubled the KoH concentration and got 5,2MMW instead of the 4,7MMW from before.