For me this all needs a bit of a retconning to make any 'continuity' sense. Ruby outright stated she used the book to "alter the deal". If the alteration is that Ash still gets his winning condition (Pablo returns, Ruby and Baal leave) but the Ghostbeaters now get dragged to hell with them, then that fixes a lot of things.
The 'happy ending' can be a false paradise, meaning the force ghosts aren't 'really' there (making them slightly less lame, if they're just Ash's paradise pop culture fed imagination), the Cabin can still physically exist on Earth which leaves the ED films in continuity (for the most part, Henrietta's still dead though) and Ash's re-appeared hand can be explained not as a time-travel re-grow, but as a gift from Ruby/Ball so they can cut it off (which they did) to make a new Evil Ash who serves as the big bad for the season.
It would take a bit of hefty dialogue from Ruby to deliver it, telling Ash everything after defeating Baal is exactly what Ash wanted it to be, she could question him on how he ever got back to his own time considering nobody can read Sumerian, for the team to all just stand there going "Uh....?" and the penny dropping. But then that essentially gives us a Season 3 which is Ash in Hell (which would be fucking great, visually and as a new concept) whilst Evil Ash (Ashy Slashy?) is wrecking havoc on Earth making everyone hate him again.
The Evil Ash thing might be a bit of a stretch, but I would give my left nut to see Ash vs Hell.