To Wasabi's point about writers and crafting stories, Tolkien initially started with creating the language and the story rose out of that. He built cultures and history and mythology around the languages he created for his ME stories. So I rather believe that while he probably altered course a couple of times in the crafting of LotR, I'd have to believe the framework was always there.
With Moorcock, I think it varies form character to character. Stormbringer was the first thing he ever wrote for the character and it was the 'final story' in that character's arc. Everything that came after happened before, and it was wildly inconsistent. the novel Elric of Melnibone was clearly intended, as an origin story, to give some form and consistency to what he'd written to that point, and it stands as a pretty solid prequel (as prequels go). Moorcock was probably more concerned with ideas rather than narrative construction, as it regarded his most famous creation. But as much as I am a fan of the man's work, I've never read that much about his thoughts regarding his crafting. I'd believe Hawkmoon and Corum, as well as his single novels such as Glorianna, were much more carefully crafted since world building are such major elements of those stories, whereas Elric was always about exploring the concept of the anti-hero as an 'anti-character' in fantasy literature. There's also a misconception about Moorcock and his feelings about Robert E. Howard-he actually loved Howard's work but was interested in making a character that was anything but an archetype in the genre. If I recall correctly, I think Moorcock was more resentful of Tolkien's work, or more specifically how Tolkien's work established a bar for the genre which Moorcock felt limited the medium as a whole.
I believe Martin had a plan with the first three books. I can see that in the ghost of the blueprint and it'd be very hard to move me off that position. But as the gaps in publications grew and grew and Martin's mind wandered more within the world he created, and with so many Worldcons to attend and TV deals to negotiate and seasons of NFL football to watch, he lost his focus and it resulted in a more ponderous and tedious follow up with A Feast for Crows. Martin has, literally, lost the plot in the behemoth he's created.
Howard, shockingly, had a pretty solid grasp on his Hyborian Age. He knew where the different countries were, knew the borders, knew JUST ENOUGH of the culture and lore of each of those regions that they served as a fertile bed for Conan stories. He could send Conan to any region to get in a fight and he could expand on that region's partiulars as the hero navigated his way through those treacherous areas. I think this is actually the best way to create episodic adventure; don't get too much into the weeds until you have to take a step into them.
As for Christopher Tolkien, he was a staunch defender of his father's creation and while this made it difficult for others to deal with him, I think that was ultimately for the best. He wasn't a fan of the films and I can totally understand why. At the same time, I believe Peter Jackson set out to do something that he believed was honoring the memory and idea of the books. It wasn't merely an appropriation of something he didn't create. Without CT around to slap Amazon around as they move forward with their TV series, I fear for that production's integrity.