Oops, yeah, a blower motor running backwards will definitely interfere with heater performance! It's not terribly likely your low heat output is due to air lock — the design of the A-body system is such that the core doesn't generally tend to trap air, as can happen in some other systems. Nevertheless, it's worth a shot: Warm the engine to full operating temperature, put the car at as high of a nose-up angle as you safely can, and run the engine at a fast idle for a couple of minutes.
You'll want to check carefully to make sure the hot/cold blend door is moving all the way to the "hot" position (and that any sealing foam on the door is in good shape) with the temperature control knob in its own "hot" position. If not, adjust the cable and/or fix whatever other problem prevents the door moving all the way to the hot-air position. Also check carefully that all the heater-to-firewall seals are intact and leak-free. Even a small cold-air leak can overwhelm the heater.
I suspect, though, that you might be removing the heater again to install a new heater core.
