One last issue: Tolkien’s elves are usually depicted as fair-skinned and lithe, whereas Marsellus Wallace is a really big black guy. Does this contradict the theory?

It would, unless Marsellus were under some kind of magical glamor to hide his true appearance. And there’s actually some evidence for this.

There’s one character who the movie clearly depicts as having the ability to cast illusion-related magic: Mia Wallace. In the parking lot of the restaurant, she tells Vinnie “Don’t be a…”. Then she traces a square in the air with her finger, and the square appears in glittering light. Marsellus Wallace is married to someone who can cast visual illusions.

But why should we believe Marsellus’ appearance is itself such an illusion? Well, in the scene with Jules and Brett, Jules puts a gun to Brett’s head and asks him what Marsellus looks like. Brett says he looks like a tall bald black guy, which seems to satisfy Jules.

The hit men try to play this off as some kind of intimidation thing, but they’re just going to shoot Brett anyway - there’s no need to intimidate him. It would only make sense if they’re actually checking how Marsellus appears to Brett - ie whether a certain illusion he’s projecting is working. When they follow up with "Does he look like a bitch?“, this is their foul-mouthed way of asking whether he looks androgynous. When Brett confirms that he looks masculine, this seems to satisfy the hit men, who then go ahead and shoot him. Unclear why they’re expecting the illusion to fail in Brett’s case, but it seems like if it has they’ll need to interrogate him further and maybe track down anybody else who might have learned too much.

How is Mia Wallace able to cast these illusions?

I would guess that "Mia” is actually Maia, ie one of the Maiar who is sent from Valinor to guide Elves and Men with their good counsel and magic powers. There’s a previous example of a female Maia marrying an elflord to guide him: Melian and Thingol. Mia is following in this tradition, and just as Melian granted Thingol’s kingdom invulnerability to attack, so Mia grants Maglor/Marsellus the ability to look like a big muscular black guy.

We actually have further proof of this in the movie. Mia overdoses on heroin and goes unconscious. It looks like she goes a really long time without breathing. You get anoxic brain injury in like four or five minutes; Mia was out way longer than that. But once they give her adrenaline to the heart (totally not recommended) she instantly and completely recuperates in a medically implausible way. Suffice it to say that she’s proven beyond a shadow of a doubt that she doesn’t have a human circulatory system, and given us at least strong evidence that she is literally immortal.