I don't have any 1st hand experience with a ph controller, but I do have an ORP controller and I have the probe in the main tank. I don't think you want your probe in the sump or close to the Ca
reactor since you will get a higher reading. You want to monitor the ph of the entire tank, not just the sump so I suggest you place the probe in the main tank.
You can just use a soft brush and rinse it with water to clean the probe. Every 2 weeks should be a good cleaning schedule.