For me, I thought of going with the "Primal Earth is Gone" theory as only two characters are "refugees" from that setting (Street-Knight and Arctouros). But, I tend to look at it like the DC Universe handles their Multi-dimensional Earths. Sometimes they're there, then they get wiped out, then their back again.
As for how they got here, while I imagine many have their own stories, for my two, I was considering "saved by an extra-dimensional being "Ala the Norn from M&M" who could only save a few people.
As for how they got here, while I imagine many have their own stories, for my two, I was considering "saved by an extra-dimensional being "Ala the Norn from M&M" who could only save a few people.