I'd forgotten that Myrtle helps him again with this task. He really can't do anything on his own, can he?

No, he can't. He's fourteen, barely half-trained, and the Tri-Wizard was designed for students over the age of seventeen - six and seventh year students. Other than *possibly* Hermione, I can't see any of Harry's yearmates being able to do any of the tasks without help. (And the other Champions get help, as well.)