How can I specify the post status of an untrashed post?

I think for your purpose wp_untrash_post_status filter will be enough. Will work with single and multiple posts restore.

Filters the status that a post gets assigned when it is restored from the trash (untrashed).

add_filter( 'wp_untrash_post_status', 'change_untrash_post_status');

function change_untrash_post_status($status){
    return 'pending';
}

P.S. apply_filtershook is used to call callback functions created by us, like a snippet above, so apply_filters will fire a callback function which we added with add_filter (As I understood wp logic right)