Send Notification after post published was working but stopped after the last WP update

Regarding Update 1. Perhaps you could compare the publish and modified dates and run your code only when they match, i.e. when the post is first published. Something along these lines,

function to_followers_only_notify( $ID, $post ) {

  $posted = strtotime( $post->post_date );
  $modified = strtotime( $post->post_modified );

  if ( $posted !== $modified ) {
    return;
  }

  // rest of the code
}