RE: If I create a dev site like this, dev.mywebsite.com
When activating a license on a domain like this, the license should activate and not count towards the activation limit.
If you find that it does, before you migrate the site, deactivate the licenses. YOu will then be able to activate them on the live site.
If you forget to deactivate them, you can deactivate them dev site activation from your Purchase History page under the Licenses tab. YOu would click the Manage Sites link for each license you need to manage and deactivate them. Afterwards your would be able to activate them on the live site.
Hope this helps!