Alt header logo and link for specific pages

You are not using all possibilities of the get_custom_logo filter. As you can see from the source not only the $html is passed to it, but also the $blog_id. You can use the latter to do a preg_replace on the link like this:

add_filter ('get_custom_logo','wpse305624_change_logo_link',10,2);
function wpse305624_change_logo_link ($html,$blog_id) {
  $homelink = get_home_url ();
  $bloglink = get_permalink ($blog_id); // or anything else, for instance the blogs's parent
  $html = preg_replace ($homelink,$bloglink,$html);
  return $html;
  }

By the way, you could also use the filter on get_theme_mod (called at the beginning of get_custom_logo) to modify the returned image id in the first place, so you don’t have to do a preg_replace afterwards. Unfortunately I don’t see a similarly elegant way to manipulate the url.