2001 Audi TT Quattro w/ 99,500 miles

I'm helping my father out with his Audi TT. He has had it shut down a few times in the past couple months. After having it for 1 week, this is what I know:

- We've replaced the coolant temp sensor
- Have NOT replaced thermostat (but I do have a new one on hand)
- Coolant levels seem to be fine
- I tried running the heater on full blast (HI w/ full fans), which I thought allowed the car to drive (forcing the 2 radiator fans to kick on)
- I have OBDII bluetooth scanner and have used it to monitor coolant temps
- Sitting idle, if I run heater at full blast, readings usually stay 155-175F (70-80C). When I turn HVAC off, temps will rise up to ~200F (93C) relatively quickly

First shut off
- Cold start, sitting idle in garage. HVAC system turned off
- Coolant temp went up to 199F (93C), radiator fans turned on and dropped down to 195F (90C), radiator fans turned back off and a few minutes later, the car shut off
- Took 20 minutes before it allowed me to start back up

Second shut off
- Driving home from work. About 10-15 minutes into the drive when I was driving on highway and it shut off. Heater was on full blast for most of the trip
- OBDII scan showed ~175F (80C) at the last reading when it shut down
- Threw a P0322 code (Engine Speed Sensor (G28): No Signal)
- Took 20 minutes before it allowed me to start back up

Originally, I thought having the heater on full would prevent the shut off. However, yesterday it shut off even when the HVAC (and radiator fans) were on.
I don't have access to the Ross-Tech/VCDS/VAG-COM tools

What is normal operating coolant temp? If my OBDII scan is showing ~175F (80C) when it shut down, isn't that low? Would there be another sensor triggering a shut down?

Thanks in advance!