Only a formal study could give you answers to percentage questions - and such a study may not have occurred.
What you can ask coaches is what percentage of them are teaching students above their own level - and it is important to distinguish between what they can do now and what they once did.
I skate in the Washington, DC suburban area, a center of government, wealth and power. Many local parents and skaters with the money for athletic lessons have an extremely competitive and status-conscious nature. They crave qualifications with words like "Olympic", "Worlds", "National Champion", "Gold", etc., and coaches come here from all over the world to meet that demand. There are a few other areas like that, but it can't be typical.
Figure skating directors and often restrict who is allowed to teach, and seek coaches whose skating and coaching careers sound impressive, as do club officers who regulate who can coach in club-sessions. Since there are such coaches available in our area, others probably have a hard time finding places here to teach, and move elsewhere.
So when I find that the majority of local coaches I've met were at least National competitors or close to it, the sample is biased.
People like Mishin or Lussi are also so atypical of the average coach that it is meaningless to the majority. There are people in any field who are just incredibly good at teaching. They typically take an extremely well organized systematic approach to teaching, and they often study the subject they are teaching so well they know virtually everything that they can possibly know about both the subject, and about how to teach and the psychology of their students. They often create the textbooks and methods of teaching that other teachers use. It is quite possible they - who represent a tiny fraction of a percent of professional teachers in any field - can teach well many students whose abilities exceed their own, especially in sports, where physical limitations and childhood background apply, regardless of how hard you try.
When you look for coaches for yourself, to some extant you have to figure out for yourself what you need in a coach. If you are a teacher, you should figure out who you are able to teach well. In the end, if you please your students, that is mostly what counts.
But I still think it is appropriate for coaches to inform students with serious skating dreams that someone else might teach them better, if true - and that you don't mind if they take trial lessons with others.