1. It's entirely random. I searched each cell only once but eventually found a Feebas spot. In fact, I think it's slightly faster to do it this way, though you may be stuck combing the entire route more than once.
2. Yup. I fed mine 3 Belue Pokéblocks and he evolved, and had quite a bit of Feel leftover. Anyone for a Smart Milotic?
3. The Feebas distribution has nothing to do with the actual Dewford phrase. Changing the phrase shuffles the Feebas distribution, but it's still random. You could use the same phrase multiple times and get Feebas in different locations.