1) Important factors that may cause mixed-ability:
aptitude
attitude
personality
motivation
learning style
previous exposure
4)
In my opinion mixed ability could help teachers and other students to share their knowledge and by doing so, they are learning something else, they are not only learning the foreign language. Students with a better aptitude can help other students ( even better than a teacher sometimes) to understand some grammar points. They feel like equals, so it is not a teacher who is telling me what to do.
I think sometimes mixed ability can be useful in the classroom and we should take advantage of it. Let's try not to regard it just as a problem.