Sorry, I am terrible at writing. In regards to using Hayden as a force ghost over shaw, I would use an older looking hayden’s face to replace hayden’s “young” face as it is right now (AKA Shaw’s ghost replaced.) As for a “rational” the view can view Shaw’s Vader’s look and sound as a result of the “lava bath” one why he looks (even sounds?) different then Hayden.
So in my mind, having an older looking hayden at the age he would have been at the time of ROTJ instead of his “young” appearance. One can think when a jedi turns to a ghost they don’t get whatever ailments they had at the time … Obi wan isn’t just a floating top half when he got sliced, Yoda doesn’t have his cross eye, etc. etc. So we have older looking hayden, how he would have looked at that time of his death without the physical ailments he had.
Of course, this is if an edit is trying to, more or less, to use keep things “connected” with the PT on having Hayden’s force ghost at ROTJ. Although I understand the change Lucas made because of the PT and trying to connect it all together, but having him young at that time just didn’t make too much sense. Now that time has passed and we have Hayden closer to the age of Anakin at that time, it would visually make more sense using him NOW to replace young Hayden at ROTJ.