Do you know what would had to have happened in order for your theory to be the correct one? The killer would have to hold her down WHILE he was making the garrote on her body, THEN he'd have to strangle her with one hand while using his other two hands (yes, you read that right!) to manually separate her labia enough to jab the end of the paintbrush in while he was doing it. AND, he'd have to achieve the monumental feat of bisecting her body so that her lower body was facing up (consistent with the penetration angle) while her UPPER body was facing down! I'm afraid you'll have to explain that one to me.