The multiverse thing was, the way I see it, a very simplistic (and messed up) rendition of actual alternate reality theory. (yes...a serious, scientific theory

)
But, Kara El, Supergirl, was from Earth one - She was killed by the antimonitor. Kara -L , Powergirl was from Earth 2 and lost her Kryptonian legacy.
Earth Prime is where the superboy that looked like the orignal Superboy (Which DC had already killed off) came from....and in the end he flew off into never never land with the Earth 2 superman and Lois after the five remaining worlds were merged.
There, Clear as mud...Right?

As for what CAN happen....anything can happen. You think you are confused? I think DC is even more confused

I just finished reading the book The Crisisis of Infitite Earths
According to my understanding that the Supergirl killed was of earth prime not Earth one Superman the elder rejected being on Earth one with the younger Superman who knows no such thing as Kryptons destruction in that universe neither does the Superman of Earth 1 know his roots so it is possible Kara can return again to be Supergirl f earth one with the alternate alias as Linda Lee Danvers as well Supermans cousin from Argo City
At least that's the way I come to understand it. Any opposing views would be welcome
The whole idea of a multiverse gives me a darn headache