new_excerpt_more link not working properly

Global $post variable is filled on by running post through the loop (the_post() method or function, setup_postdata() function). If you look at the source for Walker_Page it is not running a loop and so does not make posts’ data available through $post.

Since excerpt_more is not provided with info on the post either, you would need to track post data on your own and access it inside your filter function.