Question: how can eobard thawne be alive?
Answer: Now after killing Nora Allen again, Thawne has the power to run back to his timeline, so he does. Now since Thawne was able to run back to his own timeline, the fact that he died in season 1, is completely erased from existence. So he remains alive but in his own timeline. This is how Eobard Thawne is alive.