Owen left the cell door open, because he was fairly certain he wouldn't want to be messing with the lock when they were bringing Simms back. "No hand holding is necessary," Owen said dryly as he started to lead Simms down the hall. "This would probably be a lot more pleasant if you shut your mouth." Again, he didn't think Simms would listen, and Owen had no problem with Oliver antagonizing him if he wanted to.
His mind was racing, thinking about the possibilities and potential consequences of what was going to happen. He felt like he ought to tell Simms he was free to go, just so that he was technically following Their rules. Why couldn't they just get him in the cell again on something else? He was starting to worry that if they weren't upfront with Simms, TPTB would make mention of it tomorrow and do something stupid, like setting him free without a trial. Shit.