If they had any scenes like that, I would fully expect the use of a body double with Arnold's head digitally grafted onto a younger guy's torso. Not that I don't agree that he would need some time to get back into the shape, mind you. But for stuff like that or any stunts, I doubt Arnold will be the body used.
I like the idea of bringing back Sarah Conner to the films. Her absence was sorely felt in Terminator 3. But, looking at that article you mentioned, Silver, I saw that it was dated in 2010. I think the franchise has changed hands a couple of times since then, so I'm not sure those script plans will ever see fruition. That's a shame, because they sound like some really fun ideas.