Vacum leak, purge flow fault code P0441, EGR pintle position code P1406, # 2 spark plug misfire

All of these things could be related. When the heater controls are operated with vacuum motors the system is usually spring-loaded to default to the defrost position for safety when vacuum is low or lost. Your feet might freeze but the windshield will be clear. There is a check valve in the vacuum line going to the heater controls, but since the EGR valve is also run with vacuum, I think you should be looking for a leak first before trying to find a heater problem that might not exist.

You can spray water on the engine while it's running and still cold, and watch to see if it gets sucked in somewhere. For smaller leaks a smoke machine works best. You might find one at an auto parts store that borrows or rents tools. With it you inject white non-toxic smoke at 2 psi, then watch where it sneaks out.