[Fixed]-Save user on database (using ModelForm), only after security questions has been answered?

Normaly you need to have a boolean field which will indicate if the user was finished will all steps. By default this field will be False, and when ready set it to True. You can allow login only when field is set to True.

Another approach is using a wizard http://django-formtools.readthedocs.org/en/latest/. With this approach you need to protect user’s password somehow because it’s not a good idea to stay in the session.