I have to say, I think you’ve got this all wrong.
All the accusations about Mark Hamill could have equally been levied against Carrie Fisher: complaints about diet; snarky comments about the franchise; idiosyncratic behaviour on set and with fans.
But does this make them divas? Did Carrie and Mark seriously think that their movie-star moment had come again in the twilight of their careers, and that they could therefore start throwing their weight around?
To me, a diva is someone with a highly exaggerated sense of their own importance, and behaves badly as a result.
Instead, Mark is – and Carrie was – I think, past the point of getting affected by the hype, and both were aware of their lesser importance to the franchise. For these reasons (as well as a general cantankerousness that comes with advancing years) they treated the whole thing as a bit of fun.
The fans love it, and therefore Disney isn’t the least bit bothered.