Heater Not Working Properly

Thats right where it should be. Make sure no air then recheck hoses in and out of heater core and feel if they are hot. You have basically covered all possible problems. Unless there is no flow through the inlet hose or a blocked return which is unlikely and im not exactly sure the routing of that system. Maybe you could try removing hose from heater core and run engine and see if there is good flow through the inlet hose to heater core and then you could try blowing compressed air into the return lightly and see if flow comes out of heater core inlet, when engine is off.

Its possible. I would definitely double check and check passages when thermostat is out to make sure a old gasket is not blocking a water passage.

Check fuses. Check relays. See if power is getting to the wiper motor, turn signal switch, and fan switch.

Ignition switch or load reduction relay.

Thomas