Heater does not blow hot air i can hear fan in all speed positions but if i lower the temp range a few notches air starts to blow but is not hot enough i can hear in ducting that the vent is opening as i lower the temp range. i checked the filter unit and is clean,

check to make sure the blend door is changing from cold to hot. With the engine off and the key in the run position set the blower motor to the highest speed. Turn the temp dial to full cold and wait 30 seconds. Now turn the temp dial to full hot a listen carefully. You should hear a difference in sounds as the blend door sweeps through its full range.

If it is working and the engine is fully warmed up feel the heater hoses where the pass through the fire wall. One should be very hot and the other should be pretty warm. If one is hot and one is significantly cooler than either the heater core is plugged or there is air in the heater core causing a block in flow.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

I think you blend door is stuck.

Because the Main Gear in the Blend Motor has cracked or teeth have chipped and is now not keeping the heat/cold door to the Hot position. 3 fixes: Buy a new Blend motor, takes about 5mins to replace. Buy a new Blend Motor Gear, will take about 30mins to fix. Repair current gear with Epoxy, takes about 30mins and 12hrs for the glue to set up strong enough to use it.

Probably a stuck open thermostat or the thermostat is the wrong temp range for your vehicle.

you most likely have a bad airmix servo. this happens alot on these. they are expensive to replace too.

check dial switch, check valve that lets water run through heater core

I think you could be right about there being air trapped in the system.
Or the heater core(radiator) is plugged.
Either way you need a pressurized radiator coolant pump to put the coolant in without air being trapped.
Although,i have seen it done by jacking the vehicle up high enough that the radiator was now higher than the heater core level, and then the coolant was put in.
But this is dangerous to try.

No ductwork [to the lower vents] under the dash. They are part of the heater core / blower housing is integrated (all one piece).

After reviewing shop manual, we suspect that one of the three damper flaps is not positioning correctly; or debris has blocked one side (which makes more sense given the symptoms).

We're waiting for the temperatures to get back in the plus numbers to check out the physical arrangement.

could be vaccum leak in the a/c panel where the flappers open and close to allow hot and cold air to blow

Replace the temp control on this car, it should be electrical control not cables.