I think automating this won't work due to this OneNote API bug. Here's why.

Your main objective is to preserve the author information by injecting a new outline at the top of the page.

We can inject a new outline at the top. But that means the existing outline(s) needs to move down by the height of the new ToC outline. Just modifying the location of an outline would overwrite author/lastModifiedTime information on all children within that outline. While you can move outlines manually through the OneNote UI without affecting author info, it doesn't work the same through the Interop API.

  <one:Outline selected="partial">
    <one:Position x="36.0" y="90.0" z="0" />
    <one:Size width="309.1365356445312" height="40.28314971923828" />
    ...
  </one:Outline>

"But that means the existing outline(s) needs to move down by the height of the new ToC outline."
mmmm why?

After command

After I've tweaked it manually (which I will have to do when the ToC gets too large)

Refresh just refreshes it in place.

Well, it could just overlap the outline on top of whatever is there, like your second pic, and then let the user deal with it. Maybe another checkbox option? It would have to place it in a consistent location, regardless of where the existing outlines are located.

Is that OK?

"It would have to place it in a consistent location,"
"Is that OK?"
Yes definitely , but I would do something a bit like a post-it on a page, i.e. right justified on the right margin + a bit with a bit of text: "Make a space on the page and move this to where you want it". So you don't get loads of support questions about why they can't click on it. I finally put it indented a little so you can always click on the main page and / or the ToC.