Raisins: Luke is not dead. I think the battle with Ben was a means Luke discovered to project himself, like a force ghost, and he never left the island.
That is what happened.
Luke projected from the island onto the planet to distract Ben to give the Rebels a chance to escape. He was never there, just a very well made mirage. Next time you watch it, check out Luke's feet in the fight scenes. His steps and stances never reveals the red salt underneath while Kylo steps do. I caught that nugget on the second time through.
As to his force melding on the island, it may be a grey area of "death" where he may get show time again as a live figure or a hybrid where his force self may play a role in a fight to come though. Yoda caused lightning on the island to strike a tree; I think we can see Luke do some things likewise as well, and more things that Yoda couldn't.
In the expanded universe in the lead up to finding out who turned Jacen Solo, Luke learns to mind walk from the mindwalkers. It's similar to what Luke did in this episode of The Last Jedi. It's an ability that leads him closer to learning about Abeloth, an imprisoned darkside creature responsible for the unbalancing of the force in cycles over the last 100k years. (Spoiler w/in Spoiler: The Sith and Jedi team up to fight her because she wants to kill everything. Of course, the Sith stab each other and the Jedi in the back at times during the fights but it's a motif that recurs time and again. The Sith (the ones who know her real threat) really don't like her at all.)