Rest api naming conventions for consumer and admin
I am glad that we are finally talking about the naming conventions this totally depends on personal preference and use cases and how your project has been designed so i will keep here my views ..
Like you said all of the above seem to be good but how i would do is

GET/Instructors/all 
GET/Instructors/admin 
GET/Instructors/any other special role

you may use queries when something specific has to be done with these roles again as in 
GET/Instructors/all?credibility=PHD 
something like the above its never a good idea to show every thing on just parent calls like GET/Instructor as you said firstly it creates confusion and makes the readability of your endpoints difficult with time when the complexity of your application increases.
I am creating a REST API in python and currently, there is a route to a list of the resource instructors:
GET /instructors
But, I also have another client, used as a CRM for users who have an admin-role.
For the client, I want to display a list of instructors but with more fields and different properties.
My first thought is also to have the route:
GET /instructors 
This obviously conflicts with the route above. 
What is the best name for this route?
GET /instructors/admin
or
GET /admin/instructors
or 
GET /instructors?admin=True
I am not sure how to approach this.
Thanks in advance.