I would also assume it is possible for E. beniensis to mix with notaeus. With their rather limited range and numbers I would suspect some males have found their way to a female notaeus ready to breed.
But I am also willing to bet the habbits of anacondas and their breeding balls would make sure a small chance of success for a different species to get in.
Or we could look at it the other way and see the possibilities of the smaller notaeus males reaching beniensis females first and having success. But not knowing to much about the size diversity in E. beniensis this would be a shot in the dark theory as well.
Stranger things in nature have happened.
It would probably be safe to assume E. deschauenseii has not bred with E. notaeus or E. beniensis. At least in a LONG LONG time.
Just some thoughts : )