I am 6.5 weeks post deep-plane face and neck lift. Will this swelling under chin after necklift go away? (photo)

You had a comprehensive neck lift based on the fact that your platysma was sutured together.  It is possible some residual preplatysmal fat was left behind and sometimes additional liposuction can correct this.  The other issue is if your submaxillary glands are exposed and bulging.  These would require a neck lift revision to correct. 

At 7 weeks you are still very swollen. Give yourself ample time to heal and be patient. Remember, that skin quality plays a large role in facelift results and this can happen even when everything is done "right"

Thank you very much for your question and
photograph. At close to seven weeks’ post-op, you can still expect residual
swelling. This should slowly resolve over the following couple of months.
However, without a pre-op picture, it is hard to provide any more meaningful
advice regarding your specific results. I recommend being patient with your
healing process, giving yourself six months out from the facelift and neck lift
before judging the final results. In the meantime, though, I would bring your
concerns to your operating surgeon for his/her advice.