3. What a beautiful girl! (where is the verb? - is the girl the object? does she know the subject?)
4. Let the assignment be done (by us).
5. Let the zoo be gone (by us) (argh!!). Let the zoo be visited (by us).
6. The election law has been tried to be broken. The electin law was tried to be broken.
7. I go is wanted (by her). The fact that I go is wanted by her. My departure is wanted.
You know, not all sentences admit a passive version. Just to begin with, you must have a subject, a transitive verb and an object (these ingredients are required, but not sufficient), the subject must transmit an action to the object, which can be inverted. In my opinion none of your sentences admit a passive version. It would be interesting if someone explained one by one why these sentences do not admit a passive version.
PS Not a native speaker