The simplicity of this explanation is appealing. The evidence I can't get around is the discovery during autopsy that JBR had blood on her thighs that had been wiped off. Since corpses don't bleed, the blood would have been on her thighs prior to the head blow. This prompts the questions of how it got there, when it was cleaned up, by whom, and so on. Similarly, JBR wouldn't have bled from a paintbrush injury post mortem, although her body might have oozed blood from an earlier injury. The blood found and the blood wiped away are better accounted for by SA prior to the head blow.
I can't make out what you're suggesting about the discovery of the body. The person who turned the body over didn't know JBR was dead until that moment but then applied the garrote and the ligatures? Hunh. That doesn't make sense to me.