I have not tested this as thoroughly as others here (thank you!!) but it seems to me the fix for the stated problem in the first post is fairly straightforward.
- do not allow deletion of about topics on user admin (by site moderators/admins)
When the above is attempted, prevent it and display an informative error message. I am moving to Contribute > Bug because it is a bug that the about topics can be deleted in the above case.
I did just create a test user and used that to create a category, and then deleted the user from the user admin. It allowed me to delete the user directly even though it had a topic. The about topic authorship was given over to the system user! So at least that use case seems to be fairly safe in this regard.
I also tried the self deletion case (have to demote the user first in order to see the delete account button). Deleting that user also gave topic authorship to system. Also, I see that the default for delete user self max post count is set to 1, which means that by default it is not possible for a user to delete themselves if they have more than one topic. So we are safe here anyway.
And I tested deleting the user via user admin. Interestingly, as long as there was only the one about topic I did not get the post delete button. But after creating a second topic I did get the post delete button. Selecting it and typing in the long text to confirm then worked - I was able to delete the user’s posts including the about topic. Finally a repro! 
And finally, I was also able to delete the about topic using bulk actions. Another repro! Oops, now see that I do not have repro of bulk actions deleting About topics. It just fails to do so silently.
Given the above, we can ignore the user deletion (via user admin and self-deletion) cases because the authorship is given to system.