Major cause of A/C freeze up.
1. Low on charge due to a leak on the system. Your unit can not be recharged as it is, because there are no service ports, but that can be modified.
2. Lack of air flow, dirty filter, dirty eveporator coil, dirty blower wheel, blower motor not operating properly, loose blower wheel on motor shaft (blower wheel not spinning at the same RPM's as the motor, operating the unit in A/C Mode below 50 degrees with out a head pressure controller or a heat load with in the space that you are trying to cool.
If your unit has none of the problems above then there should not be any problem operating the fan on the "ON" position, high or low speed.
As far as humidity, the higher the humidity the more condensate you will produce from your unit this should not cause a freese up unless you have one or more of the problems above.
Here in the North/East our temps have been well over 90 degrees for some time now and we are living in our RV until our house is built, the A/C is on all of the time and i keep the fan mode on "ON" and on low speed all of the time to help with hot spots in the camper, the air is always moving and have not had any problems.
When a unit is low on charge there is not enough refrigerant in the system to obsorb the heat from the return air, this is when the unit will freeze up and if the unit is over charged then there is too much refrigerant, the unit will not cool but it will not freeze up as well and this can damage the compressor.
You may want to have your unit checked just incase.