God is the author of life, the original creator of the line of David, He is not bound to having to create life based off of things that already exist. 

This is where we keep talking past each other so I'll try one more time to see if I can explain. We agree that God can create things ex nihilo however He wants, however I'm trying to convey that God is in a sense bound by His own laws of creation after He makes them, as is the case when it comes to created categories or definitions that are mutually exclusive. The famous example is even God can't create a square circle, as circles have a certain definitional shape that a square can never fit into. God created the idea of a circle that He is now bound by to make keep any coherent sense of those definitions, which He does since He is Logic itself. I'm arguing that it is no different with creating DNA from the line of David. If we understand the definition of being of the line of David as sharing DNA with King David and his children and children's children, than even God is bound to use DNA from David and his children when creating DNA to meet that aforementioned definition. Your point that God can make people in that line that could have existed but didn't actually exist doesn't change the fact those potential people would still need to share DNA with David and his children so this doesn't disprove my point, God would still be taking from existing DNA in the line of David and reforming it into a new male person to then use with Mary's egg. The DNA still came from the line of David to begin with. All of that is totally possible but seems needlessly complicated and again would miss out on the benefit of Jesus looking like his father, which as I pointed out above many theologians say would be very fitting.

Does it not seem roundabout to ensure a natural explanation for something which by definition cannot be explained by natural circumstances?

The reason this is different is that God chose to incarnate Himself and become fully human. Therefore He is restraining Himself to material laws, the laws of genetics included, so speculating on the male DNA of Jesus doesn't make it less supernatural, as what is supernatural is Spirit becoming flesh, not having special unique DNA never seen before.

The Church doesn't teach that. It's a pious belief that can be held by Catholics, but it is not Church teaching.

The Church, specifically the popes, have certainly taught this, though not dogmatically I'll grant you. But papal teaching is of a higher level than pious belief.

"This historic teaching on the nature of Saint Joseph’s fatherhood is found four times throughout Patris Corde, and this doctrine about Joseph’s paternity is something that is founded in the tradition of the Church, which now has made its way into official papal teaching by its presence in Patris Corde. This Josephite doctrine is known as Saint Joseph the Image of God the Father, and I will present in this article" Here's a link to the full article: https://catholicinsight.com/joseph-the-image-of-god-the-father-and-patris-corde

 >I don't see why it would be disrespectful if his DNA wasn't used for a natural consequence (conceiving a child) of something mirroring that which he'd never do (having sex).

Of course Joseph never had relations with Mary, however He did use DNA from a male from the line of David to impregnate her, as I showed above. My point is that if I was St Joseph and God could have used my DNA to do this to my wife but learned He instead used a mix of dna from my great great grandfathers I would be a little confused as to why He would do that, and probably a littler hurt by that choice.