When Ray said that Kara looked just like his cousin, could it be her doppelgänger was adopted by Ray's family instead the ones from Supergirl? Meaning she's still in hiding in Arrowverse Earth 1. Or maybe it was an Easter egg to something else? Cousins in real life or something, he played Superman reference?
Find it kind of dumb Superman or Kara don't exist in Arrowverse. Krypton should still exist, and Cal El's parents would still ship him to a suitable planet, just maybe not Earth. Maybe he got sent to a planet where he's instead a villain, along with his cousin. Would make a crazy Flash episode where Superman and his cousin come to Earth to destroy it or some shit.
Or maybe he did land on Earth, but Men in Black got him instead of the Kents. Merging Earths wouldn't make sense, since everyone most likely has a doppelgänger who lives a completely different life. And if you don't have a doppelgänger and Earths merge, there will probably be some kind of singularity like in season 1 of the flash.
Typed way more than I wanted to lol.. Loved the crossovers though