I think Coutinho goes before Dembele, if there is any common sense left in our board.
Coutinho is going before Dembele. It's just that if Griezmann and Neymar both come, Dembele would have to leave as well.
Coutinho is surely gone this summer regardless of whether or not he's included in a possible deal for Neymar. Griezmann's arrival makes Cou staying at the club virtually impossible. Dembele could still stay and get decent enough playing time even with Messi, Suarez and Griezmann in the squad. If Neymar did arrive though he too would be forced out of the club.
No other forwards would get any sort of significant game time with Griezmann, Messi, Suarez and Neymar all in the squad and Dembele would have to be sold to clear his wages and recoup some of the transfer fee for Neymar.
Don't be daft mate. Griezmann is close to being confirmed and if Neymar did arrive Dembele would have to be shown the door alongside Coutinho.