Any competent criminal would make sure he did have an alibi. I simply cannot see how a man can be both intelligent and smart at the same time as being stupid?
1) Someone who commits a crime by definition can't have a legitimate alibi to prove they were somewhere else at the time they have to lie and make up a false alibi
2) Jeremy did just that and made up being at his home asleep and claimed his father called him and his claim he received such call was supposed to be his alibi.
The evidence establishing Sheila wasn't involved in committing the crimes and her body was staged to make it appear she had committed suicide caused his alibi to backfire and it helped establish he was the killer because he had no way to know about the murders unless he was the killer and would not have made up a call from Nevill blaming Sheila unless he was the one who staged her body to make it look like she committed suicide.
The argument he has to be innocent because no one can make the mistakes attributed to him is a nonsense argument.