Nothing to do with the best place to buy parts but for the Mariner you might have better luck buying Yamaha branded parts. More than likely that Mariner branded parts won't be readily available.
I am not suggesting that the switch be replaced. The switch may be just fine.
I am suggesting trouble shooting and then doing whatever needs to be done to clear up the code 28 for the incorrect position switch fault.
Then see if the throttle opening message goes away or not.
Look at the parts catalogs to see which years used your part number lower unit. Any lower units with that part number will be interchangeable.
Likely that any later part numbers that were installed on a DF9.9 will also fit.
Disconnect the pink wire from the thermoswitch. Hold the pink wire terminal to a good ground point. Horn should sound. Motor should go into RPM reduction mode.
No sensor on that model. It uses a thermoswitch. Switch closes when the motor gets too hot.
Pink wire connected to the thermoswitch at one end and to the CDI at the other end.
Some boaters want a third set of tilt and trim controls. Yamaha offers an optional switch that can be mounted at say the front of the boat. A harness runs from the switch to the extra leads coming from the 703 control box.
Main tank is on the motor. Alarm is activated when the main tank is low.
Remote oil tank has nothing to do with the alarm.
If the main tank is filled and the alarm sounds then more than likely it is the alarm for over temperature.
