Q: If someone plays Mix It All Up, and, as the Keepers and Creepers are being dealt out, someone gets the winning combo, do they win immediately, or must the rest of the cards be distributed?

A: You must finish distributing all of the cards. The person who got the winning combo might receive a Creeper which would negate their win. Consider the consequences of playing the one card, Mix It All Up, as being simultaneous. So you only check for win conditions after all of the Keepers/Creepers have been distributed.