I agree! I think that garage became a torture chamber and she was in there much longer than he said.I have always felt since there is usually truth to every lie that Sherin died earlier so that when he said he witnessed rigor mortis it was a truthful statement just didn't happen as quickly as what he said it did.
I also believe that's where he usually took her for punishment. So he could hide his abuse from his other child.
I believe he cleaned it after Sherin was gone and not before as he said.
He is an evil man and maybe a little bit insane!!