It is very possible that your water pump has failed as this is very common on the 4cyl engines from 1998-2005.
To check the flow and operation of the water pump, if you locate the coolant resivior remove the cap (making sure the system is not under pressure, when the car is cool) Have a second person in the vehicle start the car, and watch for coolant flow out of the top coolant hose. The flow should be steady, it may not be flowing very fast but it will be flowing steady. With the 2nd person in the car have them raise the engines RPMs and watch the flow out of the top hose, it should be fast and steady.
If there is no flow at any point from that top hose, the water pump has failed. This can also be checked if you remove the thermostat and stick your finger inside of the engine block and towards the passenger side and feel for the water pump impellar. Most of the time when the pumps fail they will break into many peices. The pump impellar can also fail by seperating from the water pump shaft. it will still be intact, however with your finger inside you should not be able to move the impellars.
Also if the water pump checks out, make sure your cooling fans are kicking on.
cabin heat was working great before the first initial overheating. coolant leaked out before i could test heat after i replaced the t-stat. that leak is fixed now. My main concern is if it is possible for air to be in the cooling system? If so would the return line to the reservoir still be flowing coolant? It just seems very odd for the outlet flange to split, the t-stat to go bad and the water pump all within 1 day of replacement
Well the coolant may not flow out of the top hose if there is ALOT of air remaining in the the system.
You would need an air evac tool to remove the air and properly fill the system or something similar:
Filling the system properly and making sure the water pump is operational would be the best route to finding out whats wrong with the car.