I think this is the case. I don't know how far from Melinda's his brother lived, but I would bet his plan was to use his brother and sister-in-law as alibis, and I don't mean asking them to lie. What makes sense to me based on all we know is that his plan was to stay at his brother's that night, slip out in the wee hours, drive to Melinda's, shoot her, take the gun, and be back at his brother's house before anybody got up.
He says he was sleeping on the couch, but for all we know he let himself in in the middle of the night. Maybe Melinda never even knew he was there.
I would love to know where his car was that morning - on the street, or in the driveway.
It has always bothered me that he shot her with her own gun, and then left it out in plain sight for the cops to find. Surely that could NOT have been his original plan. He had every opportunity to shoot her and leave, that night or any other night. And he chooses to call the cops and hand them the murder weapon? No. That could not, IMO, have been the original plan.