I know I see things differently than most folks, and misunderstand a lot--almost as if I was raised by weasels in some deep wood instead of by the perfectly more or less, pretty much, kinda normal folks my parents were. But! As far as Dean actually thinking Mary's dead, I'm not really seeing that. I'm seeing Dean wants to think that Mary is dead, and refuses to accept anything else, not the slightest bit of hope. Because he knows what happened to Sam in Lucifer's hold, and it was horrible, and I'm sure he can't bring himself to imagine his *mom* going through that, bad-ass bitch she may be.
I could be wrong, of course. It could be that Show intends Dean to actually think she is dead, but that's not the Dean that lives in my head, so....
It's season 13, so I intend to blab about any darn thing that pops up in my mind whenever it pops up. Feel free to join in and please feel free to tell me why I'm wrong if you think I am. I'm not that delicate! Well, I am, but what the hey, you can't hear me crying.