WMG / The Books of Magic

Thomas the Rhymer is really Tom Lehrer.
They're both songwriters named Tom, and Thomas the Rhymer's song "The King beneath the Hill" fits the rhyme and rhythm and general style of Tom Lehrer's "Irish Ballad" (aka "Rickety Tickety Tin") perfectly.
  • Doubtful, as Thomas the Rhymer is a genuine mythological figure, and Gaiman has used the same tune with different lyrics elsewhere (notably in Marvel1602).

Harry Potter is Tim's counterpart in an alternate universe.
A very obvious guess that even Peter Gross (artist and eventually writer of the ongoing title) believes, given that Harry briefly appears (unnamed) in the 75th issue, holding his Hogwarts letter and running through the wall to Platform Nine-and-Three-Quarters. It makes sense, even with the differences between Harry and Tim, and their worlds — after all, many of the alternate Tims we saw in the series were radically different from "our" Tim, and their worlds were very different too.

Dream changed history to allow Titania to pass as the Queen of Faerie
In many of her other appearances, such as in The Sandman, Titania is depicted as having been an ancient, goddess-like figure. Certainly much older than a girl born in the Middle Ages. For example, she remarks in "A Midsummer Night's Dream" that she recalls Shakespeare's story having been sung in Ancient Greece by "a boy with a lyre" (implied to be Dream's son Orpheus).

But in the Sandman story "A Dream of a Thousand Cats", Morpheus implies that humans came to rule the world (formerly ruled by cats) by retroactively changing reality through sharing a common dream. It is also shown that DC Comics super-villain Doctor Destiny was able to use a ruby containing some of Dream's power to control physical reality in the waking world. Destruction also asserts that each of the Endless defines not only their own area of control, but also its opposite. Thus Dream, as defining everything that does not exist or never happened, also defines reality.

The Endless are not technically supposed to couple with humans. If they do catastrophic things can happen. But Dream has indicated in the past that he could work around this by making at least one mortal lover (Nada) into a goddess if she agreed to be with him. Helping Titania become more than a mortal human could have been a step towards a relationship. Whether they were lovers, or merely close friends, Dream might have changed reality for Titania in order to make her into the Fairie Queen she has become. This could also account for her enormous magical powers, which dwarf those of virtually all human practitioners of magic. It would also explain the confusing spread of possible timelines leading to Tim's existence, since Dream might have overwritten various histories to produce the one in which Titania became what she wants to be.