Reinhardt, try as I may, I struggle to understand the point you may be trying to make...
I agree that "Engineer" is a too widely used term, and thus subject to conveying a lack of clarity. For myself, I like to show the respect earned, by using it to only those who have earned the privilege of being accepted into a professional engineering association. I am not such a person. Otherwise, we also have technicians and mechanics - which I am both, with the skills and signing privileges to prove it.
But, for my experience, and knowledge of many test pilot's backgrounds, a test pilot can come from a wide range of life paths, with none being better than another. Nearly all of the test pilots I know, including myself, were drawn into the role by invitation, rather than application. For me, and many other test pilots I know, it was suitable flying experience on the type, and role, with a strong mechanical/systems/aerodynamics or certification background, which resulted in their being invited into the job of test pilot.
It can also be a matter of the body of skill and knowledge being deep or wide. I know some test pilots who only fly a few types, and freely admit that they would not even touch certain types or tests, where others (like me) are wide but not deep in their role. I "check" test fly many different types, with many different modifications, but if a very substantial test program is needed, I will probably hand the program off to a more qualified or experience test pilot, with a skill set in that specific realm.
This may be a rather "old school" perspective, but it is what I know from my corner of the industry. The result is a great challenge in defining a prescribed path to end up a test pilot, it just seems to happen to those who can fill the role as needed. I think it is very much the employer's to choose what experience and qualifications are needed, not so much the applicant's to anticipate.
Therefore, for anyone aspiring to the job, the best which can be said would be to gain education and experience, along with piloting experience, and then be sure to be employed somewhere that flight testing is being undertaken, so you can express your interest.
Perhaps more will work toward, and apply for this role than will be called, but we will happily encourage those who demonstrate an interest, as new test pilots have to come from somewhere....