Okay, let's pretend that the past can be changed and that paradoxes are somehow not an issue. How is the sigil going to be so precise as to get Henry to arrive one hour early when it couldn't even get him to the right generation of his family?
Instead of pointing this out to Henry, Dean says that he and Sam might cease to exist if the past is changed. "Listen, I understand that this is not your idea of a happy ending, and that you're disappointed that me and Sam are mouth-breathing hunters, but you know what? We stopped the Apocalypse!" Henry still isn't convinced. "If this works the way I planned, then there won't be an Apocalypse to stop!" Yeah, no, the past 7 seasons are already on DVD, buddy. You can't change that. Dean seems to be thinking it over.
Larry tells Sam that Abaddon can't be stopped, but gives him a set of coordinates to which the key should be taken. "Throw it in, shut the door forever," Larry says. "Why would I do that?" Sam asks. "Because it is the safest place on Earth," Larry explains. "Warded against any evil ever created!" Is it a Nutelleria? I bet it's a Nutelleria. Sam is horrified at the thought of losing access to all that knowledge, but Larry insists it must be done to keep it away from Abaddon. As soon as Sam mentions that his brother has the key, Larry's wife reveals herself to be Abaddon. She knocks out Sam and goes after Larry with a knife. Blood spatters across a picture from their younger days. The Men of Letters should have used anti-demon tattoos, huh?
Later, Dean gets a call from Abaddon on Sam's cell phone. She had more success figuring out modern life than Henry did. She's also wearing the comic book shop girl's outfit, even though they were totally different sizes. She offers to trade Sam for Henry and the key and gives him a meeting place. Henry still insists he can go back and fix things. He turns back to the blood sigil he's drawn on the door and begins chanting again. Dean grabs him by the neck. He should have been like, "I'm gonna punch you so hard, even your grandchildren will feel it! ...OW!" Alas, we don't get to see exactly how he subdues Gramps, because it's time for another commercial break.