Id get Rheingold / ISTA+ before spending 1 more dollar on this issue.

If you really want a new battery Id at least test:
voltage with car on and with car off at battery terminal
how much voltage is lost in the battery from one day to the next.

The dsc error does come on whenever the battery is unplugged but If dsc error comes on while car is running and not on start up after the car sat over night it would non conclusively probably point to something other than the battery.

if the battery was really bad, wouldn't it trigger the error at night when voltage is lowest instead of when the car is running?

As far as the steering angle sensor, why not test it before replacing? there is a steering angle reset function in rheingold where you set the steering to 0 and you can see the sensor reading. There is also probably a way to test voltage at the sensor terminal

the problem with replacing parts right now is that if it turns out to be something else, the joke is on you.