The AHU should have had face & bypass with the HWV open 100% below 36* OAT and then modulate the F&B to keep discharge temp. We have this setup in NE Ohio and it works well.
But it sounds like you don't have that.
I think your problem is when the valve modulates towards closed, the outlet side of the coil is tripping the freezestat, (if it's a running trip).
Have you checked the water flow thru the coil? Is it at design?
Is the coil piped properly? (you'd be surprised at how many problem jobs we find that are piped backwards)
Is the airflow at design?
Adding a run-around pump may help, but it's expensive. If you can, I'd add a temp well sensor at the coil outlet and bring it into the control scheme as a low limit function.
Adding glycol may be a problem if the capacity is sized tight.
What is controlling the VFD?