I'm not sure about any of this. In the post above yours the TS describes his situation. He's dealing with a shorter guy and his straight goes either over the guy's head, or into the top of it. Recommending the straight at that point sounds like a good way to break his hand. It also suggests that the guy might not be stupid, and is going low with his head to avoid that very punch. Besides, as you suggest in your post, securing positioning FOR a punch insures it's safety, thus, throwing an uppercut is no more risky than throwing the straight if the proper angle is secured ahead of time, or the punch is timed well. Roy knew he could throw uppercuts in that bout, that his opponent had no idea how to defend against them. So he did, that's why it worked, it wasn't a chance punch.
To the TS, you can slip overhand punches just like straight punches. You may need to bend your knees to assure the punch goes sailing over whichever shoulder you prefer. Depending on which way you slip, is the counter you use. If you slip left, when you pull back to stance you bring a hook with it. Either to the body or to the head (if he's low, throwing to the body is smart because it'll hit SOMETHING. Whereas a high hook may miss everything). If you slip to your right, his inside, there will be a nice opening for an uppercut to the body. If you accompany either slip with a step in the same direction, you'll completely have either the inside or outside angle. What I would not recommend, is giving ground (moving back), unless you plan to use it to bait him and try to beat his punch the way Nelson did with his uppercut (always a good punch against shorter opposition). If you just back up, it says to him that you fear the punch and he can use it to MAKE you back up when he wants. That's not discouraging.