Maybe he coshed Novy with the gun, prior to smothering or strangling her. So he had to get rid of the gun due to her DNA on it.
The children were in the house. I think it is possible that he decided to commit a 'quieter' murder, and then told them that mum was 'sulking' in her room and don't go in there. Possibly sleeping with them in the spare room to ensure they didn't go into Novy's room.
Then he later told them that their mum had run off because she didn't love them enough to stay.
I am also thinking that all the to and fro of his older daughters the next morning could have been to get Novy's children out of the house so he could do his big clean up of the crime scene.