In some cases the traction control button is just a user interface to appear like the stability control is turned off. In reality a gain is applied to lessen stability control. This is a recent change and will become more predominate.
Depends entirely on the make and model of the car, but for the most part it will let the driver do what they need to do at the track.
So you've added a turbo to what was once a naturally aspirated engine? And it has no O2 sensor to monitor AFR? I've never heard of modifying a modern engine like this so I'm genuinely curious. I've tuned a late 90s engine similarly but I had some help. I had a wide band O2 sensor right after the exhaust manifold and a pressure transducer in the cylinder.
All cars have and need an O2 and it is still used once you modify the car. You use the wideband to get a readout of how lean or rich the car is at certain RPMs which takes that data, puts it into a program and logs the data real time to give you an idea of where you are at. Here is a pretty rough explanation of what a fuel map is and looks like inside the program.
https://hackedecu.com/understanding-fuel-and-ignition-maps
When you say you alter the code, how exactly are you doing this on their OEM ecu? They have their own programming and most I've dealt with are encrypted. Most communicate via the CAN bus or FLEXRay or something. So do you have this equipment? Or do you alter some other way. I've attempted the same with another ecu and was told it would take 100 years to crack it (It was from a manufacturer on the list).
99% of the time I do not use the stock ECU, I use a aftermarket ECU that gives me the freedom to mess around with everything easily. Stock ECUs are extremely restrictive and don't allow you to mess with the data very much at all, you can in some cases trick the ECU into doing things but it's a crap way of tuning a car. For the most part even dealerships do not have the ability to mess with the fuel and ignition tables, they can usually only re-flash ECU software or use it to pull logs off the ecu such as driving habits or past engine malfunctions. The only time I'll use a stock ECU is when I take it apart and chip it myself, I'll de-solder the stock chip and solder in my own that allows me to upload my own modified software.
I used to race at the track every sunday up until I was in my early 20s, fiddled around at shops before I realized I needed a steady income that allowed me to have a paycheck in the winter as well. Joined the Air Force and now I turn wrenches on jets instead of cars now. I still mess around with my toys on the weekends but don't allow myself to go crazy like I did when I was younger.
In my glory days my fastest time was a 11.4 in a shitbox eagle talon, which was at altitude in Colorado. I've worked with subarus, hondas and DSMs, a little dabbling with Mazda rotaries but nothing crazy, those were definitely the least fun to work on.