Here's my input about that one supernova disproving the young universe:
First of all, I think that the in transit theory is false and un-Biblical, simply becuase God would not lie. There is a much better theory out there by a guy named Russel Humphreys(did I spell his name right?), and I think it explains why we see old supernove in a young universe. In his theory, the way that he explains how we can see light from distant stars is by saying that in the beginning the whole universe was a huge white hole and Earth was close to the center and the far away stuff was not. And as you know, time virtually stands still at the center of a white hole, and so the things farther out would have aged by billions of years and yet on Earth only a few days would have gone by, thus some stars would be able to go supernova in just a few days(within the creation week I assume?), or so it would seem from Earth.