Hey Joanie this is the first time I have heard of lemon juice being a "diuretic" do you have any references on this?
I think freezing might just be OK and it would be kinda convenient to freeze in 2 tbsp cubes to make each drink but I'm with you that I also put my lemons in the fridge and they do seem to "last forever". I know I have over bought and have had a bunch left after finishing the MC and if I recollect correctly it seems that they were still good 4 or 5 weeks later.
I have never made my lemonades by the drink so the cube freezing thing is a moot point for me. I also believe that while doing it by the drink might be the ultimate way that the advantage of mixing it bulk for the day and having the lemonade at the ready to chug it down when you get hungry and or feel a little weak outweighs whatever small gain one might get of doing it by the drink to help keep us on tract and not caving to hunger or weakness.