Gas and Oil Home Heating Furnaces - how to troubleshoot aprilaire 112

My folks have an Aprilaire 112 humidifier that is not working. A couple of things I know already. There is a relay at the furnace that appears to be functioning since I do get 24v coming into the humidifier when the furnace is running. I also know that the outlet that the humidifier is plugged into has power. Based on diagrams of te unit, it seems that the problem could be either the solenoid or the fan.

I have not removed the unit yet since the last person to work on it used silicon caulk to seal it against the furnace. Does not seem right since you need to be able to remove the unit to replace the water panel - but that is the way it was installed.

I do not hear a fan running in the unit nor do I hear a solenoid click when powered on. There is also no water draining or leaking through the unit which suggests that the solenoid is not working. Is the fan dependant on the solenoid to operate?

Is it worth replacing the solenoid or should I replace the entire unit? I think I saw the model 700 as the suggested replacement.

When I cut the silicone sealing the humidifier to the furnace, what do use to seal it again? Foam tape? more silicone?

While looking for parts for unit, I saw a mounting adapter plate. Would that make reinstalling easier? I guess I could caulk the adapter to the furnace and then just use the regular clips to hold the unit in place against the adapter. Am I right?

those humidifiers are not that expensive, i would replace it if over 6 or 7 yrs old. after mounting the plate you would still have to seal. use silver tape and mastic, covering only the non removable section.