Buddhism, or at least some types of Buddhism, don't believe in a "supernatural creator". Are you saying Buddhism therefore isn't a 'religion'?
It depends on whether they believe in any kind of supernatural force/soul/spirit. If they do I would still call them a religion, otherwise they I would consider that kind of Buddhism to be a philosophy or ideology.
What if a person believes in the existence of something 'supernatural' (ex. ghosts), but doesn't believe in "a God/creator". Are they atheist?
In the strictest definition theism is only the belief in a god or gods, so technically a person who believes in ghosts but not in a god is still an atheist. Which is why Atheist is an incomplete definitions of what most people who would call themselves atheist consider themselves to be. Atheism is only one facet of what is truly naturalism. The term atheism is simply so ingrained in the culture that it is easier to keep it and not bother calling ourselves naturalists, especially since in our society it is implied that one that calls them self an atheist is not a supernaturalist. For example I would not be tempted to call Deepak Chopra an atheist, only as a matter of convention.
So, if that definition is prescriptive, go ahead and lay out the set of attitudes that all atheists share. Go ahead and layout all the beliefs that all atheists share. Go ahead and layout all the practices that all atheists share. If you can't do all three then I'm afraid you're flat wrong. You are correct about those two facts, but neither of them mean that atheism is a religion. It just means it has some things in common with religion. But sensible people don't completely equate two things just because they share one or two things in common.
Some of them do. Some don't. However, all that proves is that being atheist doesn't entirely preclude being religious. It in no way whatsoever supports any claim of atheism BEING a religion.