I think that thing in the middle of the scrollwork is more a medallion than a knob. I think you would normally use the key to pull the door open. We have buffet that, although there's no ornamentation on the door, it has no knobs or pulls.
As to drawing the scrollwork, I think I would start drawing the perimeter and then use a method similar to the one that Marcus linked to. I would simplify it a great deal, though. By the time you get all the ornamentation drawn the model will be huge even by your standards. π I expect the upper and lower scrollwork on that applique are the same just rotated 180Β° I would make a component of just one and use that.