I entirely agree with Sgt Jones that Greg cannot be the culprit unless the police have made a total pig's ear of checking his alibi. If he went to Sheffield then returned then went back to Sheffield, that too would be take him through many verifiable points - and makes him way too vulnerable if any tiny detail goes wrong.
But what he may have done in the four hours is to "sanitise" the scene of the crime for some quite different motive. In other words he might have removed relevant evidence in order to conceal some detail he didn't want to come out.
He might regret doing so later but decide to stick with his original story rather than face the music for misleading the police enquiry.
I'm not saying this is probable, but it seems possible. Sanitisation of the scene of a death by family members is not unusual.
On another issue, I'm wondering whether to take the "no evidence of sexual assault" with a grain of salt. The police are clearly giving us the minimum possible info and they could perfectly well claim that even if she had very recently had sexual intercourse, this might have been consensual.
