No, the idea that we're descended from off-worlders is completely poppycock. Almost every aspect of it is contradicted by well established science, unless you are one of those who clings to the idea humans have only been on Earth for just 6,000 years. However, it is as refuted as that idea. (And even creationists swear that this is the homeworld of humanity.) Though it does fit better with a twist of Mormon theology -- and Glen Larson was a Mormon, after all.

It's not impossible that aliens (not human) visited us in the past, though it grows more and more unlikely with time. For example, they would have had to clean up anything they did on the moon or Mars, since we have now examined those closely enough to have seen any significant structures. Cleanup is of course an odd thing to do for a people losing their technology in a war. And visiting aliens is not the plot of BSG. These guys are humans. (Or rather, I think they are humanoid machines who think they are human.)

As to what Earth would be when they get to it, the writers could write anything. Dead planet. Thriving planet. Dead and reborn planet. However, as far as we know somebody rescued Starbuck and immediately took her to Earth. This suggests that they are from Earth to me, or at least that it's not dead.