I'm not sure that I would try the buttonhole attachment on a 206 (and like machines) mainly because of the issues you have encountered. If I was really desperate, I may try setting the stitch length as close to zero as possible and go very slowly. That would almost defeat the purpose of using the attachment. The uniqueness of the 206 machines, I would worry about messing up the machine for regular sewing. I know that some have a designated machine for buttonholes - like a 66, 99 or 15.
Janey - Neat people never make the exciting discoveries I do.