I personally think a Watsonian argument for a young Force ghost Anakin is irrelevant. If JJ wants the Sebastian Shaw version of Force ghost Anakin, they’ll make a hamdwavy explanation for it like they did to explain how our heroes could see Starkiller destroy Hosnian Prime from across the galaxy. I think a director’s concern for canon is low-priority. The story group can just say, “Well, Force ghosts can take whatever shape they want, and Anakin decides to be his older, redeemed self at this particular moment, so it doesn’t contradict the SE ending of ROTJ.”
I can see JJ making this decision. But it does come with a few cons when you look at it outside of his own personal preference.
First of all, they might need to use CGI to recreate a version of Shaw-Anakin, like they did for Tarkin in Rogue One. To me, the biggest issue would be that anyone who grew up watching any post-2004 versions of ROTJ are used to seeing Hayden-Anakin as a Force ghost, and probably wouldn’t even recognize who he was at first.On the other hand, having Hayden come back as Force ghost Anakin has some benefits. He will be immediately recognizable as Anakin to even the casual Star Wars fan. They could just film Hayden as he is now, since he has not aged dramatically in the past decade+ since filming Revenge. While some fans might rolls their eyes at the addition of Hayden, a lot of younger fans who grew up with the prequels and Clone Wars will likely appreciate the addition, especially since online sentiment towards the prequels seems to have turned around slightly in the past few years.
To me, audience familiarity is strongest argument for Hayden to play Force ghost Anakin.
I definitely can see arguments for or against that choice. I do think if you try to look at it objectively, having Hayden return as Anakin seem to be the most likely option, if Force ghost Anakin even appears at all (I would surprised if he did anything more than pop up briefly). Even if JJ might initially want old Anakin, I could see how he might be persuaded otherwise for those reasons. Sorry if I’m just reiterating what has already been said.
I want Hayden, but honestly I think JJ won’t show Anakin in any form because of the issues you describe.
If the man has a mental breakdown at the sight of Podracer flags and makes his CG guys redo those shots because “my movie isn’t about podracing”, then he would never in a bajillion years put Hayden’s Anakin in his movie.
And it can’t really not be Hayden from Disney’s perspective, for the reasons you mention.