Thank you for this question. Here is what I was able to find on this that felt helpful.
According to uslegal.com “The saying ‘possession is nine points of the law’ is an old common law precept that means one who has physical control or possession over the property is clearly at an advantage or is in a better possession than a person who has no possession over the property.”
The following is from the November 1890 Christian Science Journal:
Our Master taught his students to deny self, sense, and take up the cross, … Is sense “denied” by fleeing from it? or by overcoming its claims? Realization, reflection of Principle as idea, is the “closed door” of the senses, and the only “denial” of the senses. Will not every one bear witness that the claims of sense are often loudest when the eyes are closed? Is the Cross “taken up” when the claim of illusion is acknowledged as true, and the admission is “signed, sealed, and delivered,” the witnesses to it being the senses themselves? … “He that hath ears to hear, let him hear!” is always the word of Principle.
“Turn away mine eyes from beholding vanity.” (Heb. emptiness, nothingness.) The basis of this supplication is recognition of sight as spiritual, now. Is not shutting the eyes to pray, or to realize the Truth of Being, to start on the opposite supposition — that it is material? Does not this concede possession — the nine points of the law of mortal mind — to the enemy of Good?
… Our enemy has been slain ; we are not in bonds; we are freedmen of Principle; “wherefore, brethren, we are not a bondmaid’s children, but the freewoman’s.” With her freedom Christ made us free; “Stand ye, then, and not again with a yoke of servitude be held fast.”
We do see and hear spiritually, and we do not see or hear materially. This is the standpoint from which to encounter and destroy the illusion. Science and Health is full of this.
Then from the January 30, 1904 Christian Science Sentinel:
The old condition may seem worse under the light than when it lurked in thought unexposed, but in all reason it is not. Mrs. Eddy says in the book “No and Yes,” p. 33, “Exposure is nine points of destruction.” To see clearly how evil has entangled one’s career, opens the way of escape from the unhappy condition; and that which throws light upon the unrighteous complications, is in every case a blessing.
From all this, I feel that the “nine points of destruction” come from exposing each point of the error that is claiming possession of us using what we learn in Christian Science.