If the flow sensor is bad I would expect you would be getting a no flow error, not low salt. My first guess is that the cell is failing; this could certainly give you bad salt readings (mine gave incorrect low salt readings just before dying). If the cell is bad, raising the salt level to compensate for the low reading isn't going to help.

How long have you had the cell? While cells are expensive, over the long haul I don't think they are much more expensive, if any, than manually adding chlorine. It's just a question of paying as you go or all at once every several years, IMO.