Re: Teaching as a separate skill.
I agree that personal skill does not necessarily equal teaching ability. This is from personal experience as well as observation/reading etc.
There are people who may not be able to do things to a certain level for reasons including their own physical challenges. And yet they can teach others to do more than they can do themselves.
I have experienced aikido teachers who can demonstrate significant skills themselves, and yet their ability to train others to similar levels is, by observation over many years, limited.
On a different tack, there are people who are "naturals" and themselves have problems understanding why other peopl can't do stuff. Plenty of stories on this front.
After all, Usain Bolt's coach can't run as fast, and yet he can help Bolt do it...