That edge looks fine. You want the thin part to be long enough to lightly beat along the full diagonal edge of the braid; the rest of the taper just helps you smoothly open the shed.
This short video has some excellent views of the motion of takadai braiding from multiple angles (starts around 53 seconds in):
To unbraid, you pick up the last tama that moved, pull it across to the other side, follow the thread back through with the sword to recreate the shed, and then pass the tama back to the top. It’s very tedious, and I wouldn’t do it unless the mistake was very recent. Better to just insert a marker thread, continue braiding correctly, and keep it as a reminder.
- This reply was modified 3 months, 2 weeks ago by J Greely. Reason: correct start time