Is this double chin 12 weeks post deep-plane facelift acceptable/normal? (photos)

You likely still have swelling.   If you developed any post surgical seroma or hematoma this could take  a few months to settle and cause prolonged swelling in the neck.   After full healing if you have persistent fullness in the neck then a limited direct excision and revision platysmaplasty may be required.

The fullness in your neck could be due to enlarged salivary glands, or enlarged digastric muscle.  It is not uncommon to "unmask" these structures once the overlying fat is removed during a face-neck lift.   An experienced and talented surgeon can properly assess the patient pre-operatively, and be prepared to deal with this issue during the surgery.Only an in person evaluation can determine what remaining structures are causing the residual bulge in your neck. Any further revision surgery should await up to a year after your last face lift. 

This is a good question. The healing time after surgery is usually a minimum of 3 to 6 to 12 months for swelling to resolve. The body heals along this time line and swelling and pain resolve accordingly. Looking at the photos you posted, it appears as if your neck could be tighter at this time frame. A good Facelift  includes the Neck and it needs to look as good as the Face.  Many surgeons do not master the details required for the neck to look sharp and tight.  There are several surgical maneuvers that can be performed for the neck to look very good. This includes a tightening the platysma muscle and creating a sharp Cervicomental Angle.  This usually involves cutting the platysma muscle in a certain way to delineate the angle of the neck. Also, if the neck is full, the tissues need to be examined appropriately.  Sometimes the anterior belly of the digastric muscle creates a fullness that can be eliminated surgically.  Adding this to the procedure will help create a sharp neck.  This is a especially important in a Male facelift because the fullness is usually present. I always remove part of the digastric muscle in men because if helps to sharpen the angle. These concepts are unique to creating a youthful neck.  The Neck is as important as the Face and should not be overlooked.  In your photographs it appears as if you still have fullness present that may remain as you heal. If so, then you may need a revision. At the time of surgery, the surgeon should see the result on the table and check for the proper result.  If not, then continue to create the best neck surgically at that time.  I hope this helps.  If it continues to remain full, you will need a revision. Sorry that is not what you wanted to hear, but ...... Good Luck.