Based on those instructions .... this pool was a time bomb. I think you just got unlucky being there when you are.
Review the Pool School discussion about the CYA / FC relationship ... this is not understood/accepted by the pool stores, so it is not surprising that the owners do not understand it either.
If all they have been using is tablets, the CYA could be very high, which requires a much high FC than 2ppm. See Chlorine CYA Chart. Although, they have apparently accepted a leak, so that may help keep the CYA level reasonable.
Not sure about the scare tactics regarding the pump ... $8000 will get you an entirely new pool. A motor/pump is closer to $2-300.
Review the Pool School discussion about the CYA / FC relationship ... this is not understood/accepted by the pool stores, so it is not surprising that the owners do not understand it either.
If all they have been using is tablets, the CYA could be very high, which requires a much high FC than 2ppm. See Chlorine CYA Chart. Although, they have apparently accepted a leak, so that may help keep the CYA level reasonable.
Not sure about the scare tactics regarding the pump ... $8000 will get you an entirely new pool. A motor/pump is closer to $2-300.