Adams is known for writing the screenplay for the film in the many attempts it was shopped around to make the film. I have no reason to doubt that the majority of the screenplay for the film was made by him before his death.
Considering he wrote other interpretations of his works, most notably the radio drama, I see no reason to doubt the official story. BTW for you Youngins, the Hitchhiker's Guide to the Galaxy radio drama is pure Kino. Listen to it sometime.