So, I guess right now, the way I see that this went down would be...
She leaves him and takes the girls with her. This infurriates him. He starts calling and threatening her. She is scared and lets her parents know how scared she is. Even writes those letters about how scared she is he will kill her.
His threats are getting nowhere so he concocts this plan.
Leaves to go get her, stops at Wal-Mart somewhere in Ohio, buys a shovel, gloves, a cap, and a tarp.
He has a throw away cell phone (wrapper left in his truck) so he was most likely calling her and making threats as he was on his way.
Stops somewhere else, digs the hole.
Stalks the house, sees her come outside by herself to mail that letter.
Grabs her, kills her.
Stops back by the hole he dug, buries her really quickly.
Drives home so he's there when LE comes looking for him.
My question would be, why did he know throw all the evidence in the hole.
Like the gloves, the cell phone wrapper and stuff?