I have a thought about Sam showing up. Not something I hope happens or hope does not happen or expect the writers to use. Just a reasonable path.
What if physically leaping into a pit that leads to hell does not qualify you for hell? Two canonical ways of getting to hell are 1) being sentenced to hell as punishment 2) agreeing to a very explicit contact with a fair amount o ceremony. What if those are the ONLY ways into hell?
Jumping into hell with Lucifer in your body does not qualify as either one. And we more or less have the word of God that Sam was forgiven past sins. And I know that jumping into a pit leading to hell is a pretty strong form of consent, but going to hell when you are not sentenced there seems not so much a matter of consent as a matter of making a formal agreement to go to hell. (I'm going to handwave away the last minute demon blood slurping on grounds that's the Supernaturalverse's God and Angels are pretty strongly in the "end justifies the means" camp.)
So Sam's body went to hell. Lucifer in Sam's body went to hell. Sam stayed here, and thus is a ghost. I have no strong feelings either that this is the path the writers should take nor do I predict that this is the path the writers will take. But as good an explanation as I've thought of or heard to date.