Would a pre-made (commercial) electronic conductivity probe be allowed for use in the salinometer, as long as it is only a sensor component?

No, a pre-made (commercial) electronic conductivity probe is not allowed for use in the salinometer, even if it is only a sensor component. If you have the conductivity measurement then all the student would be doing would be converting that number mathematically to a percent solution. This doesn't meet the event rules that the student must BUILD the device.