To allow BW images you need to add the color option to your price list and you must have a color filter that matches your color option. This option is added by default by image store and all your customer needs to do is select the right color.

Here is a post with a bit more information Color filters

Is it possible to send download links manually?

The sort answer is yes, but it may be difficult to generate a secure link manually, it requires the encoded image id, color code, noce data and image size. The link look something like this

imstore_admin_url/download.php?_wpnonce=$nonce&img=$encoded_image_id&sz=$size&c=$color_code

The link is active only for 24 hours and less you changed your nonce experition time.