Did Coop know Sarah was Mother/Judy? where did he hope for when he was standing at the door with Laura. To kill her? That seeing Laura's alive  creates some kind of reaction by sarah? Coop was on the hunt for Judy more than 25 years. But i'm asking myself if he really did know judy would be opening the door. 

I also think, 25 years ago sarah wasn't possesed by the judy/mother. I think it happened when 'the experiment/judy/mother  escaped the glassbox,and after sarah visited the grocery store