Added a check for the single.php template, if it is absent, index.php wi...

Single.php is not a requirement for a WordPress theme. If a comment is not accepted, and if the single.php template is not available, index.php should be used. It is the only alternative you can be sure that will exist. For my theme, this works fine. It only has an index.php.
I think I see why you are removing these. From the docs: "The local user ID on the client site of the content author." So, for user registration, this doesn't look really relevant. user_register does pass the ID of the newly registered user though.