I will try to keep this short and simple, long story at the bottom.
1) Originally had r410a Carrier Comfort package installed. (FX4D air handler with TXV. 25HCC530A300 Comfort heat pump.)
2) Contractor swapped out the motor from the air handler for a variable speed motor from the FV4CNF air handler. He also swapped over the Smart Board as well.
(Contractor says that the only difference between the two air handlers is the motor and the control board. So now I should effectively have an FV4CNF air handler.)
3) Contractor also installed an Ecobee smart thermostat.
Here's where I'm at: The contractor refuses to hook up the dehumidify output of the thermostat to the Dh terminal of the air handler for dehumidification. His reasoning is that this can ONLY be done if the heat pump is a 2 speed unit, whereas mine is only a 1 speed heat pump. He's telling me that if I do this, the air handler will make the fan speed too slow and it will cause my coil to freeze up when the air handler is trying to dehumidify. He's telling me it will freeze up just like when the filter is clogged and the airflow is too low. Is this nonsense? I have read two threads where I think people are doing exactly this here and here.