I'd say that everything in the Silmarillion and Unfinished Tales is canon, and anything in The History of Middle-Earth that doesn't contradict them is also canon. So Beren DID die, Lúthien went after him, begged Mandos to release him, and they returned to life as mortals, albeit with incredible lifespan, and went to dwell in the Land of The Dead that Live. That's how I think the tale ended.