Deep customization of the comment form?

This is hard. Output buffering could solve that:

add_action( 'comment_form_field_comment', 'ob_start' );
add_action( 'comment_form', 'wpse_83898_replace_submit' );

function wpse_83898_replace_submit() 
{
    $html = ob_get_clean();
    # do some magic
    echo $html;
}

Just an idea, not tested.