I don't believe for a minute that he told SW he wanted to separate. I think she told him because something happened before or over the weekend to trigger her response and it could be she learned of his affair. CW seemed perfectly happy to have a wife and kids and a girlfriend on the side.
But if SW separated and took the kids, he'd be out of the house, paying child support for three kids he would see maybe once a week and he sure wouldn't be welcome amongst "her people" at Le-Vel. I've always believed Laci learned of Amber Frey and that's what triggered her murder.
CW dug the grave before SW every came home. He had to because there wasn't enough time that morning. Hence, it was premeditated.
I feel like they wouldn’t charge with first degree murder unless they had some pretty solid evidence to prove it was premeditated. Seems that if there was any thought that it was “an act of passion” or whatever they call it for a spur of the moment thing that they would try for 2nd degree as that would be easier to convince a jury of without the proof. That’s JMO, I am not super familiar with how that works.