Let's say I question if ever there was a real person behind the Orpheus myth. We can look at the historical record for clues and from there try and make up our minds about how flimsy or strong or non-existent the evidence might be. But trying to argue Orpheus never descended into the underworld to rescue Eurydice is neither here nor there when it comes to proving the original contention. This part of the story is myth and separate to the question of historicity. I feel some of this falls under that banner.

Similarly, if Mark never mentions the census or the trip to Bethlehem and Luke does, it's overwhelmingly likely that Luke fabricated his addition to the point of it almost being trivially obvious. But that in itself does not repudiate the original 'source' of Mark. I put source in inverted commas, because the reliability of that source is up for debate, but this part does actually have a fascinating mystery alongside it. If we consider there was no historical Jesus of any kind, then it's still kind of interesting to consider why there is this Greek text from a few decades after about this otherwise obscure corner of the world.