What I consider the strongest evidence against the Marcionite priority, obliging me to introduce ad hoc hypotheses to corroborate it, is the titulus crucis.
Is it anti-marcionite? Remember that Mark considers Jesus "called king of the Jews" as the true Jesus, not the marcionite Jesus Barabbas.
Hence if the Barabbas episode is an anti-marcionite interpolation, then accordingly also the titulus crucis. The function of the titulus is to proclaim: this guy on the cross is NOT Barabbas. This guy on the cross is not the marcionite Jesus.
But then an interpolation leads to another:
Since the titulus is strictly connected with the Pilate's action ("Are you the king of the Jews?) then also Pilate would be an interpolation.
The Passion Story is full of anti-marcionism in all the gospels, included *Ev.