Well, fuck it, my take on it.
There is a single Wonderland, but potentially infinite "instances" of it. Those instances have the same "base" characters (Mad Hatter, March Hare, Queen of Hearts etc.) each representing aspects of human mentality, plus groups of "adventurers" physically or mentally close to each other. All such adventurers ("Alices", perhaps. ) are either insane or dying in the real world, the Wonderland being the last escape of the sane mind against being lost in madness or death. It is possible to travel between seperate instances of the Wonderland using mental links, as most humans are connected in one way or another.
Once a person is there, there exists two possibilities. He can turn into a permanent part of the dream, losing bits of his personality as a prominent characteristic takes over, as with our unfortunate group (the red one got taken over by bloodlust, green by the desire to be liked. ) As such, a new Alice could face his long-dead love in the Wonderland, with a charming face but a personality twisted beyond recognition by the dream. Though, not all such ex-Alices are unpleasant, just as how Wonderland denizens can be pleasant.
The other is not giving in to the dream and successfully escaping while fighting former Alices and more bloodthirsty or just plain insane of the denizens, perhaps in a way described in >>4238982 . Then the character in question makes a miraculous recovery and wakes up, his adventures nothing more than a bizarre, blurry dream, though his tales will still be told in his instance of the Wonderland.