Will there be a scar after TMJ surgery?

Little if any. Most "tmj" surgeries are performed via an "inverted gull wing" style incision utilizing a small crease in front of the ear. This incision is the same one used for face lift procedures, so it is very cosmetic in nature.
Yes. There probably will be a small scar. However it is usually hidden by your hair line.
Yes. Yes, but usually insignificant. Arthroscopic surgery - very small scar. Arthroplasty (open surgery) - bigger scar but in front of the ear - like a facelift scar. Very cosmetic.