While I agree with most of your post, I differ on my interpetation of the struggle. He got in (through the garage, ruse like picking something up, or simply by knocking on the door at a time that she was sleeping soundly.) A knee to the abdomen would double her over, then a choke hold while dragging her upstairs. The blow to the abdomen would leave her struggling to breathe, and a choke hold is very difficult to escape.
I think he planned to drown her. But I think while he was trying to stopper the tub and run the water and hold her... she recovered enough to struggle. At that time he struck her in the head with something.
I believe there was some water in the bottom of the tub, but not enough to have drowned her unless she was unconcious. Thus the combo of the head injury and inhaling water caused the sinuses to be full... but positioning over the side of the tub and probably decreased respirations prevented water from making it to her lungs. In the time that elapsed between her death and her being found, some water would have most likely drained out. But not a full tub. So maybe an inch or so of water in the tub?
One of the strongest things that convinces me that DrewP was involved in her death was how she was found. When she wasn't available to get the boys, DrewP didn't contact family in search of her. To me, you would do that before you jump to the conclusion of something wrong in the home. And DrewP arranged for witnesses (not family) to be present when she was found. I really really hope that the prosecution points that out.
If DrewP had contacted family in search of her, the family are the ones who would have most likely found her. And DrewP wouldn't have an excuse to be present and controlling the scene.