Replacing the default content created while the site creation

I can only answer your first two questions as I do not know anything about multisites.

The default comment and content in the ‘About Us” page is not coming from the theme files, but rather the database. They are the default posts/pages/comments accessed by the backend not by the theme files. To get to this content and replace it login to the admin via youraddress.com/wp-admin
and locate the about page in pages > about us. The comments are under the “Comments” section. Simply delete the page or replace the content.

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