The ONLY eligibility for NFCU is through personal or family military or DoD involvement. So, no you cannot join or qualify without a family member.
In addition, you must be directly (first degree) relation to the family member which must be a parent, grandparent, child or sibling. The only other way is to be of the "household" of a member, meaning you live with them. This means if a cousin is "eligible" then you need to get them to join, then their grandparent who is also your grandparent to join, then you could join.
For clarification, NFCU requires that the person be CURRENTLY tied to the military or DoD to be eligible. This means they must be in the military or working for the DoD "now" or be retired (paid retirement), not just former or "veteran" status. I am a veteran and was not qualified. My father in law (active) had to join, then my wife, then me.
Lastly, based upon what you said PenFed's reasoning said, they are not counting your AU accounts as references. While those accounts give you a nice FICO, upon "manual review" by the Loan Officer, he/she has determined that your majority of tradelines and limits are AU, which is not your financial responsibility and therefore they will not count them as a credit reference (reference means an account they can refer to for YOUR past payment history).
You can try to use Alliant, and POSSIBLY get some type of "instant" online approval based upon your FICO. However, all auto loans will have some type of manual review, because they all most always involve at least some type of verification and also require closing and title documents. So even an initial preapproval could be pulled back upon an LO reviewing and seeing that your CR is mostly AU's.
AU's give you a FICO boost. But it gives you virtually NO help if the application requires a manual review, UNLESS you have adequate "personal" credit tradelines. If you are relying upon AU's as the majority of your credit and limits, you will have limited opportunity to get meaningful credit.
I would suggest that you contact Alliant or another CU (PenFed is super-prime, ultra conservative as previously posted) and open a savings account. Drop your auto down payment into savings. Put a direct deposit from paycheck to it. Open a checking. Begin doing your primary banking with them. If you cannot get a loan initially, use your savings to obtain a secured personal loan, then pay it off in 3 to 6 months. Get a credit card from them. You need to build a "relationship" and history with the CU. In time you will be able to get all the credit you need (so long as you manage wisely).
Good luck. 