A couple of thoughts.
First, understand that punishment at nonjudicial punishment is, by definition, not severe. If you're actually guilty of the misconduct, then you escaping court-martial is, in and of itself, a benefit.
So consider rewording your appeal to focus not on the punishment but on the quality of the evidence presented.
Also, I understand that hearsay is admissible at article 15
. So arguing that they should not consider hearsay is, in my opinion, a waste of time.
That said, the reputation for honesty of the individuals who provided evidence against you is relevant. If you have any evidence that suggests the folks who provided evidence against your are liars? That would be highly relevant. If you have such evidence, I would focus on that in your appeal.
Also, evidence of your good military character is relevant. Presenting statements from folks who know you well, and can testify to your military character, can also be helpful on appeal.
Please let me know if you have more questions. I am happy to help if I can. Otherwise, please rate the answer so I may get credit for my work.