1) Yes, you can include Highslide in your API. For commercial users you'll need a commercial license.
2) Even though the JS is loaded from a single domain, it will appear on multiple websites, so you need to get the Unlimited license.
3) Use the "Buy now" button on the http://www.highslide.com
website. You will be given a license key that is used to identify you and your customers as authorized Highslide users.