Comments.php is not getting called on main blog page

Comments can be enabled on the archive page by setting the global variable $withcomments to true before calling comments_template():

global $withcomments;
$withcomments = true;
comments_template( '', true );

As Michael pointed out in a comment on the original post, comments_template() does not return output when $withcomments is true, or in a number of other scenarios:

function comments_template( $file="/comments.php", $separate_comments = false ) {
    global $wp_query, $withcomments, $post, $wpdb, $id, $comment,
           $user_login, $user_ID, $user_identity, $overridden_cpage;

    if ( !(is_single() || is_page() || $withcomments) || empty($post) )
        return;
    ...

comment-reply JS

Normally, you’d want to include the comment-reply JS where comments are being handled (single posts), but the the built in comment-reply JS script does not seem to be intended for use on pages with multiple posts, so you’ll need to roll your own solution instead.