How do you know Scully didn’t really see Mulder? There’s precedent on the show for having visions of someone who is dead. In fact, I think this is probably a callback to the scene in “Beyond the Sea” when Scully sees her father in her living room right before she gets the phone call telling her he has died. [[SPOILER:
She finds out Mulder is “dead” right after this too]]
It’s open to interpretation, but I would say that Mulder is trying to get back to Scully, just like Cas is trying to get back to Dean. (That is to say, I don’t think it’s all on Scully’s end. Which addresses what I think you were implying about Dean being less hung up on Cas. Though I’d also like to point out that Dean himself thinks he is imagining seeing Cas at first. What does that say about Dean’s state of mind?)
The compositions of the scenes are strikingly similar. Scully and Dean have the same hopeful then heartbroken reaction to this “vision” of the person that they love whom they fear is lost to them forever. Reyes and Sam are both concerned and ask them if they are all right. In both cases, there is a strong bond between the two pairings that draws them back to one another in a supernatural way.
So, nonny, to conclude: I don’t think Scully was dreaming. And even if she were, the way the scenes play out and the emotions in Scully/Dean that are exposed when Mulder/Cas disappear again are the same.
I’m not saying romantic love, but I’m saying romantic love. Also, aliens. ;)