Can a court martial "un-do" a retirement?

Short answer - Until there has been a discharge, you've received your DD-214, and you've received the final pay & accounting, you're still likely subject to the court-martial jurisdiction. Talk to your detailed defense counsel, but the best course of action is to lay low and keep off the command's radar until your discharge is final.

I tried a court-marital similar to this about 25 years ago. The DD 214 is effect on mid-night of your last day of service. Look at the date of the DD 214 as to when you separation date is. If your separation date passed, then you are clear. If the DD 214's effective date is in 60 days, then they can pull you back. Check with an attorney who knows how to read these.

By the facts you presented you are not retired, so there is nothing to "undo." They can call you back until midnight on the day on your DD 214.

There are also some recent cases in which discharges completed erroneously have been voided post the discharge date.

Yes, your Command may take action that could impact your medical retirement. You should, in the short-term, remain in contact with your IDES, civilian, or military attorney. You remain on active military duty until 2359 on the retirement date listed on your DD-214. Nonetheless, even past the DD-214 retirement/separation date, Article 2, UCMJ (10 USC 802(4)) suggests as a retiree you may still be subject to discipline under the UCMJ.

When I was a Trial Counsel with 2nd Brigade, 101st Airborne Division, I was able to pull a Soldier back onto active duty from a medical retirement based on fraudulent discharge. Seek a competent Military Law Attorney's advice.

Short answer, you are subject to the UCMJ until 1) DD214 completed and you pass that date, 2) cleared installation and outprocessed 100%, and 3) final pay and accounting completed (generally weeks after you've been out. If you were subject to the UCMJ and Court Martial charges filed, that by law stops all DD214 and outprocessing until the CM is dismissed, dropped, or completed. When I was TDS I represented a client who was under similar situation, awaiting CM trial but passed his ETS date and extension. He had a completed DD214 in hand, and had outprocessed and cleared 100%, and was at home. We argued the motion to dismiss with the Court, and lost. Judge cited the rules stating that he CM stops all outprocessing, DD214, etc. Come to think of it, this issue came up a handful of times, and never fell in favor of the accused. In your case, they can simply extend to for the purposes of a Court Martial, typically in increments of several months, and void your orders to retire. Sorry for your situation. Sounds like they are being evil over this UA. You need to aggressively work with TDS or a civilian lawyer to get them to drop this and let you medically retire. Good luck.