I am not sure of prune juice. Prune juice is a laxative all by itself. If you mix CO and prune juice, it will be double the laxative effect. You can try any other juice like apple or whatever. I think the idea of adding some juice is just to mask the taste of the CO. I just use bottled juice. Only once I squeezeed fresh juice of oranges.
Yeah, lemony water would be perfectly fine. I do that too sometimes. lemon is a good aid in clearing mucus and cleaning the intestines.
I try to drink lemon+honey in 1 liter of lukewarm water every morning. It helps in keeping my bowels clean.
Good luck with your cleanse. I might do it next week or so.