After reading all of this it seems you already have good advice, there is little I can add.
Your mix is not your problem as said, but you should get back to 50/50 ASAP to avoid corrosion. (Plain ole H2O does work the best though for cooling)
Is it possible that you are overfilling the car? This is the most common cause of puking, not so much backpressure, but for what it is worth you should only fill it to the top row, not the top itself.
I had the in line flush tee on the heater hose as well, I just hid it under the air cleaner. When refilling your car if you leave that cap off until you see coolant start to flow from it and then cap it and continue to fill, you will insure that very little if any air gets into the system. (without one leave the top hose off until you see coolant start flow)
The factory gauge is just a reference, without numbers you can't really verify that your thermostat is working properly. You can check it on your stove though.
You really should put a fan shroud back on. Not only does it help protect the hose, but without it your fan is really just blowing the warm air in the compartment around. The shroud makes sure the fan draws the outside coller air through the radiator though thus aiding in cooling.
Does your bottom hose have the support spring in it? If not even though it worked fine before it could randomly collapse without a spring and this could be a possible way that air is entering in if the radiator is trying to pull water through a closed hose.
The lack of a heater system is not the problem, I dumped mine years ago. I even went so far as to run one long hose in and out of my firewall so it still looks stock. I hate to see it replaced with a little bypass hose instead.