All in the above list, with the exception of nutritionists (if they are indeed degreed in the field), have little or no scientific knowledge of nutrition. Veterinarians, at most, usually have only one course in nutrition during their schooling. This is not to say a person cannot be self-taught, but then their expertise should be evidenced by written materials and actual experience in the nutrition field.
Even "nutritionists" are no guarantee of competence. Nutritionists are responsible for the feeding of instant potatoes, Jello, canned meat, and diet pop to people in hospital beds. They are also behind the myth of 100% complete and balanced pet foods that have caused immeasurable death and suffering in animals.
It is a simple matter for any pet food producer to talk with professionals in some field, or pay them a fee, and then say the product is "developed by" them.
So, the claim by pet food companies that they are backed by this or that expert provides very little assurance. Also, it is a simple matter for any producer to talk with professionals in some field, or pay them a fee, and then say the product is endorsed or developed by the expert.
The proof of competence lies in the product itself and the competence of the producer as evidenced by their written materials. The most important credentials are those of the person(s) who are in charge of the company and who make the final decisions as to how the products are made. Anything else is just name-dropping.
See How To Choose A Pet Food Company.