P.S. Am I the third reader or a lucky forth?
After a few rounds of swinging the code axe, I am cautiously optimistic I have an answer. Be warned. What follows involves code modifications to your theme’s functions.php and for older posts, some clearing of stuff in your database. Now that there are maybe three readers left (Hi Tom!), here we go.
This is another great deep dive Alan. I have not quite got to the stage of carving things out in the backend, but have started down the road of a Child theme thanks to your help. I am interested in adding your solution at that level. It really bugs me how the default oEmbed bakes in the parent post.