Okay, I've been thinking about it and imagine this:
UNIVERSE B (Post-Scratch) -Beforan ancestors exist around the late 1900s/early 2000s, which coincides with the Alpha adults -The Condesce that Lord English sends from Universe A shows up, kills The Constern and takes over Beforus before doing all her weird Crocker Corp. shit on Earth -Sends prisoners from Beforus to Earth to get rid of them from her "perfect troll society" -The Imposter (Karkat) ends up on Earth among all the other prisoners, who are then integrated into human society because most of them aren't aggressive/dangerous (i.e how Karkat got arrested for negligence and not something like homicide, etc) -Alpha Dave meets Karkat -??? -Profit -The Condesce floods the Earth and everyone dies
(I think this would work as a slight offshoot from the canon timeline, since it still gives the 400ish years for the Dancestors to be born and end up existing around the time Dirk and Roxy are alive on flooded Earth)