I not sure if SG killed her or if he did he if did it alone, but I know either way he is the key. I think he brought her jeep back and parked it. I'm not sure what the relationship was between them to be honest. I don't think there was a gun in her back imo. I do believe she knew he was married and I do believe she knew wife has cancer and was hospitalized. I don't believe it was premeditated. I do believe it was a crime of passion...in the least heat of the moment. I do believe it happened in that house. I do believe he has hidden the body. I'm not sure where I stand on the drug angle. I will say if they are involved I don't think it was pot.
It's not a laughing matter, but your last sentence "I don't think it was pot" makes me laugh. No kidding. Pot smokers don't have enough motivation to carry out any laborious task... Especially not discarding someone.