Do I have to use Zwift for Spindowns or can I use the Wahoo App?
You can use the wahoo app. Spindown data is stored on the trainer, not the app.
Agree with David, and from what I remember from the past people had issues when trying to do spindowns/calibrations through Zwift and it was recommended to only do spindowns via the manufacturers app (if they have one).
Wahoo app works great.