TMJ is short for Temporo-Mandibular Joint. If you are having pain or other trouble with your TMJ, the increased jaw tension or joint stress would not typically cause your face to sag. It is unclear to me what deformity you have now is due to the trauma you describe 31 years ago. Surgery to fix a deformity due to "medical reasons" (such as trauma or cancer) may be covered by insurance, but surgery to improve facial appearance alone would not be covered by insurance.
The one muscle that a SMAS-type facelift may tighten is the platysma muscle. The rest of the SMAS facelift does not tighten muscles, but rather tightens or lifts the tissue above the muscles. The facelift can help the sagging appearance of the neck, but would not be expected to relieve your TMJ problems. Even if your platysma muscle was contributing somehow to your TMJ problem, lifting it or tightening it would not necessarily help your symtoms.
Also, a facelift would not address your teeth grinding (bruxism). As a side note, Botox has been used for relieving jaw tension and helping with teeth grinding. I personally do not inject Botox for bruxism, so I do not know if that is covered by insurance. The best way to find out is check with your insurance provider.