Humidifier and preventing ice formation
Ice in your fridge with a RH of 40 is not because of your humidifier, unless you stand with the door open for long periods of time.
I would expect your defrost timer in the fridge was not is not functioning or a fan was not properly moving the air.
Any moisture you put in the intake of an ac system it should by design take out l. Don’t introduce into the intake , the purer water is the faster it will freeze.
If you want to humidify air the moisture should be introduced after the air is chilled.
dehumidification Is a side effect of air conditioning  this may be why you are struggling to get the Humidity up all the moisture you add is being removed with each pass through the evaporator (cooling coils in the air handler).  Try adding moisture at the vents so the moisture air is pushed around or add a humidification system after the coils then all the vents will vary the humid air.
One thing I almost forgot to add the type of humidification can cause worse allergies if there any impurities in the water I would call good RO water the same as store bought distilled probably 19 meg ohm per centimeter.