First there is no excuse for refusing an order unless the order is: illegal, unsafe, immoral, unethical, or unsafe. It is one of these items then you better be prepared to defend it with facts. Stating no in the presence of others to a senior leader is most likely not the best course of action. Perhaps attempting to pull him to the side and explain your concerns might have been a better course of action but that is hind sight at this point.
Have you already taken your ETS physical. I recall in some cases Soldiers were exempt from certain activities to prevent possible injury before ETS. This prevented the Army and/or VA from becoming liable for unnecessary injury. Now this was years ago and I do not know of a regulation that covers this activity but you might check some of the medical regulations or ones that deal with physicals.
If you are recommended for an Article 15 typically you will not be held past your ETS and it takes a lot to get a dishonorable discharge. Odds are you will leave on your ETS date (you might pull extra duty until the day you leave) with an honorable discharge unless they decide to chapter you which would most likely be a general discharge (the odds of this are rare but possible). The downside is you might leave 1 or more stripes lighter depending on if they give you a summary, company, or field grade article 15.
You might find the following post useful:
How can I convince my chain of command to drop an Article 15?