While I understand you making an IG complaint, did you also submit an appeal of the evaluation? That was the correct way to appeal the evaluation. In that process you could have submitted statements of witnesses as part of your evaluation appeal. Unfortunately there's some time limits. You might still try to do a corrections board petition. You should be able to argue that any delay in proceeding was as a result of the delayed IG.
www.court-martial.com; www.court-martial.us.com; email@example.com 703-298-9562, 800-401-1583. Answering your question does not create an attorney-client relationship.
Your only real possibility of a timely remedy is to write your U. S. Senator or Congressman. You should include signed statements from the five witnesses that were ignored by the IG.
Additional facts are needed to fully answer your question. You may still have time to appeal the evaluation. Additional information is available on my website. Feel free to call me to discuss your options. The consultation is confidential and at no cost. Best of luck.