I also served and will throw in my 2 cents. An illegal order would be something akin to a lack or authority from the orderer or an overreach of the rules of engagement or standard military conventions. If you are ever asked to do something extreme, the secdef or secretary of state or their deputies intervene directly. The president has a direct line to the secdef at all times. Remember that he removed the secdef a week after the election - I imagine the acting secdef, a specops guy, is much more willing to execute trumps orders. I have no idea what the IA does or its limits, but if there is an impeachment but no trial - which it looks like will happen - he still has all of his authorities. I'm not sure if pence is the one who can invoke the 25th, but if the cabinet wants to do it, they technically can at any time, although it was written only to replace the president if he became a vegetable.