Heater not heating; Temperature Gauge stays at about halfway.

About a week ago I took my 2007 Lexus G350 to the dealer for a Factory Recall (replace the VVT, or something). I had recently bought the car when I took it to the Lexus dealer for the recall, so I asked them to inspect everything else and let me know if they saw any other areas or items that I may need to keep an eye on. They noted that everything looked great and the car had gotten all scheduled maintenance from prior owner, and just to continue getting the scheduled maintenence.

About 1-2 weeks after this, I was driving the car on the highway, I pulled off on the exit ramp and stopped at the stop light - when I stopped, the car sputtered out - it just died, and my Check Engine lights came on. Specifically it said "CHECK VSC" and the Check Engine Light. I was about 2 blocks from my home so I started the car back up, it started, but obviously did not sound right, it was almost sputtering out every couple seconds. I baby'd the car the 2 blocks to my home, where I later popped the hood and discovered that the Tube from the Water pump was disconnected (if looking at the car from the front, it was the tube coming out of the pump on the LEFT side). The clamp for the tube was still on the tube, it seemed that it had simply "slid off", and I could tell that engine coolant had splattered all over everything when that happened (which I presume happened when the car died out when I exited the freeway).

Later, after the engine had cooled, I re-connected the hose and filled the Pump with Engine Coolant directly. I turned the car on and let the air make it's way out of the hose/system (ie. I burped it). This resolved all the check engine lights I was seeing and the car was functioning normally as far as I could tell, not over-heating, the AC worked, the heater worked, the temperature gauge was staying under about halfway between Hot and Cold maxes.

I drove the car for about another week, and I wanted to just check that hose to make sure it wasn't coming loose again, and sure enough, when I looked, the hose was not snug up where it should be, it was nearly about to come off again. I again snugged the hose up and burped the air out, and again no check engine lights came on the next time I started the car. However, I turned the heater on yesterday, and the air never got hot, it was freezing cold and stayed that way for about 15-20minutes of running the car. It also seems to me that the temperature gauge gets up to halfway very quickly (within the first few minutes of driving the car), and will remain there for the duration - never really going any higher.

I also notice that my fuel consumption has been better (ie. I'm getting more miles to the gallon, which I believe is another sign that my engine is "running hot", although again, no check engine lights and the temp gauge stays at half-way).

I'm not sure what the problem is here exactly and I'm worried to run the car any further until I can know for certain that it is not over-heating. Is there anything else I can do to resolve this? Any information you can give would be extremely helpful and much appreciated.

Thank you.
Jeremiah