To answer the second part of your question, people can indeed simply share the downloaded game, since Itch.io doesn't apply any DRM. (You're free to implement your own scheme.) Sharing just a link is still easier, of course, but I'm not sure how download links work for buyers who aren't logged into Itch. For those who are logged in while making a purchase, the game is attached to their account, so sending the link to someone else probably won't work. However, people can always share their login details with somebody else, however unsafe that is. At most we can ask them not to.