Do u think if belphegor stayed around (in Jack's body ofc) long enough, Deancas actually could have a reconciliation/united front arc?
If there's one thing about Dean-Cas, it's that they are always reconciling when they shouldn't be. The script for 15x01 still strikes me as the bodily intent to want to CLING together in their grief, despite everything. ("Thanks." -> "Welcome." Combined with Dean trash-talking Chuck as a way to bandage his wounded pride AND signal to Cas that he's clear-eyed about Chuck now.)
With Belphegor, it's trickly. It's clear Dean doesn't WANT to like Bel, but seeing Jack's body walking-and-talking seems to delay his grief. Furthermore, as uncomfortable as it is, with respect to possession, Dean (and Sam) are both going to react to possession of a loved one differently at the junction than Cas is imho... They've had years upon years of dealing with this issue, at least indirectly. I think they can dissociate from it a bit more.
And even though Cas has never been on the Other End of a loved one's possession, even he seems to come around to tolerating Belphagor more than you would expect. There's raw pain there, sure, but there's banter, too. And honesty. Cas is able to TELL Belphegor what's eating him: "CASTIEL: You're wearing Jack, who was like a son to me, like a coat. Every second in your presence is intolerable. It's an abomination. You're an abomination. Is this it?"
That emotional honestly is bitterness and grief, but it strikes me as having roots for something of a John-Sam emotional honestly sphere, too. Given that Belphegor behaves a bit as an echo of Cas (sucking up the souls a la Godstiel), I think this lens actually has legs to carry it into the strange zone of "In core ways, Bel is more like Cas than Jack is," territory. Which could be REALLY fun.
Sorry, what were we talking about? I got carried into Bel-land. ANYWAY. I think Belphegor had counterintuitive leg up when it comes to actually GROWING on all of them.
It's the face.
But yeah, I think there are plenty of arcs WITH Belphegor in them where Dean and Cas could have reconciled. Not only are they weirdly prone to adopting unlikeable creatures to fight beside (and especially if those creatures prove themselves), but they're actually STILL reaching out to and checking in with one another the entire time in-script and on-screen. They only really rupture when Jack's body is destroyed (NOTE: This bookends Dean's actions of running closely with Cas + Jack's body in 15x01 and his script-only words about bringing Jack back) and Rowena's death (so interesting, because Dean is DEVASTATED by losing her, even though he'd been panic-driven cruel to her only moments before).
NOTE: I do think Belphegor would give as good as he gets. Once he finds out about Jimmy Novak, he IS going to point out the hypocrisy, which I think would lend a lot of John-and-Sam flavor to the dynamics.