Excelsior is definitely not accepted in CA for initial licensure. They don't have official policy on Excelsior for endorsement - just that they evaluate the educational background. Word is that they are not accepting Excelsior RN's with experience from out of state. You could probably call them and get a definite on that.
If you are not staying in CA, it is accepted, either initially or by endorsement (reciprocity), in all other states. It is also acceptable in all 50 states if working for the VA.
I think the thing overall with corpsmen challenging nursing boards is that so much of it is based on training that can't really be totally translated to civilian equivalents. A lot of your knowledge also depends upon where you served, hospital, ship, Marine Unit. You will have learned a ton of things that nurses never need to know and nothing about such things as Nursing Process, Nursing Care Plans
and Nursing Diagnosis which is probably what is hanging you up with trying to challenge the Boards. I am sure you possess the clinical skills that are needed.
One other suggestion I could make is that if you are eligible to try again to pass based on your experience, try doing Excelsior anyway, but apply to take the exam based on your challenge not the Excelsior education. Even if you just do the Nursing courses, it should give you what you need. It's really self-guided study for the exams, not just taking them cold.