He didn't KNOW they wouldn't suspect him, he believed they wouldn't. Like any criminal, there would be no crime IF criminals though they would be suspected from the off. What was he supposed to do after the two shots happened? He had no choice but to go ahead and make sure that he put himself some place else when the murders were supposed to be happening - hence the phone call.
He didn't know, but he couldn't turn the clock back, he just had to crack on. They were too busy to suspect him from the off.
Sorry but that doesn't ring true. You cant just say that he
believed they wouldn't suspect him. So he totally botches her 'suicide' - and his mental response to this is merely to believe that the police
wont suspect him?
It makes far more sense that the police didn't suspect him because:
(a) They were fully aware Sheila was the killer
and
(b) They were comfortable with the fact that her second gunshot wound wasn't foul play.