Does a writer need to provide for a literary executor as separate from a testatorial executor ?

Probably. I'm not a lawyer, nor do I play one on TV, but I'd think that dealing with copyrights and trademarks is a specialised job, and someone who's good at divvying up savings accounts and making sure the silverware goes to the favourite niece isn't necessarily going to know what to do when Hollywood comes calling for the film rights that the old curmudgeon wouldn't let go of when he was alive.