Changing fonts and stripping tags can be done with a mySQL query. You'll just need to figure out the right regexes to search and replace.
For everything else, it sounds like you would be better off updating the single.php template. Social links, ad codes, all those sorts of things are better managed in a single place so that if they ever change, you can ...
You can do it with WP-CLI. wp post create will create a post.
wp post create ./file.txt --post_status=Publish
where file.txt contains the content you want to add (in HTML format).
To post it multiple time use a for loop.
You can use wp post update if you want to update existing post.
First step check your post ID using
wp post list
Now update the ...
If the file you want to attach (eg. footer-registered-include.php) print/display some text, this text will be displayed in the time you include the file.
To assign to the variable the content displayed by the included file you should:
turn on output buffering
contents of the output buffer and end output buffering