Here are the responses to you queries.
1) DevTest 9.0 – What type of license is consumed for REST Invoke API calls? is it Runtime license?
As a consumer it utilizes Runtime User license. For instance, if you are invoking REST API calls from a CI process its considered runtime license consumption.
2) Does DevTest 9.0 adjust the license usage automatically depending on the permissions we increase/decrease for a role?
Yes the ACL controls the license in use. It's better to not assign multiple roles to a user to avoid confusion. The "highest" user type takes precedence overall all lower user types.