Its difficult to understand... i think the weasel just simply took the form of the actor after 'his' death to suit the girl's preference.
The weasel had always done things for the sake of the girl. I believe all her customers was the weasel under different forms, making her really popular in the brothel.
She then became interested in this actor. The weasel impersonated the deceased dong ze and became the actor's benefactor. Testing 'his' character and eventually scheming to have 'him' betray the girl with the study abroad pretense.
The actor seemingly drowned her and then left for 'his' trip abroad.
The weasel may have orchestrated the actor's death as the whole abroad opportunity was fraudulent to begin with. It may have been retribution for what the actor has done to the girl and a pretense for the weasel to impersonate the actor and be with the girl.