Not a real answer to your hardware questions, but could you rig up a simple manual switch that lets you select from among 3 or 4 voltage-divider arrangements? Whenever I've used a ph meter I have always known whether I had a strong acid, something more or less neutral, or a strong base before getting an exact reading. If you have a clue as to what your are testing beforehand and can anticipate a reasonable range of output, you might be able to tailor the voltage divider to the output and get a lot more out of the 10-bit ADC.