Is it possible to retroactively apply his possible CTE to the murders and robbery?
He was found not guilty of the 1994 murders, so there's nothing to retroactively apply there. He plead and was found not guilty and did not use any temporary insanity pleads to do it.
But even outside of the law, it's a tough sell that CTE made him murder his ex-wife, her friend, and cut her head off, and that he should be considered innocent and allowed to live a normal life after that. Aside from the domestic abuse charges, OJ lived a successful life in the public eye and there hadn't been indication that he was suffering mental illness. While people suffering from CTE might be able to hide the effects, other noteworthy people to suffer from it and take drastic action -- like Junior Seau who killed himself -- have a lengthy record of suffering, depression, abuse, and other indications.
There's also no indication that OJ suffers from CTE. And especially no connection between CTE and murder.