An excellent question, whatever our individual views it was indeed something and something significant in effort, presence, and especially action.Because the English word for nation and even country is very broad, I hold the CSA was nation or country. The only thing it was not was an international person with recognition.
Look forward to a discussion on this.
Along your lines, I think nation and country are indeed very broad terms that might fit. Though I might personally use a narrower definition of nation or country on the grounds of being more useful.
In which case I would propose a comparison. If the US lost the American Revolution and re-submitted, would we still think of the "United States of America" as a Nation, just a failed rebellion, etc?
Depending on the answer to that question I would most concisely label the Confederacy as either
a) An attempted Nation
b) A failed Nation
Personally I would lean towards a (though understand b as a possible fit also). I think a more useful definition of a "Nation" or a "Country" is something that must meet some more specific narrowing specifications. Probably one of these:
1) It exists outside of wartime, basically it can be observed as a "nation" as a peacetime entity for a certainly length of time, say for at least a year or so outside of major military conflict (but also existed in total for something longer as well).
2) It exists and sustains for a certain length of time, probably at least 10+ years. In this case it wouldn't have qualified under #1, so an alleged Nation that started in war and ever only existed in war is more of a military entity rather than a Nation IMHO.
3) Recognition as a Nation from the Nation it split from.
4) Recognition as a Nation from multiple key Nations. So it's home Nation denies it, but it's peers or superiors in relevant international context recognize it.
Basically meeting a subset of requirements I'm not listing here... such as size, believable government organized, etc. Then it must meet at least one of these preconditions before before it grows from a "Rebellion" or one of many terms applied to a military organization or other political organization into an actual Nation.
Sustained rebellions are indeed a thing and we don't consider all of them "Nations" despite their claims. I think if the Confederacy is a "nation" from the more specific context that means many other entities were/are indeed Nations and that creates a lot of noise and then forces us to define a subset of Nations that fit my parameters within that overly broad category (basically the actual useful definition of Nation in how we typically think of it).
Again if one says it was a nation then it was a failed nation. On the other end it really was a Rebellion or again some other term applied to an organization of it's type, then I personally prefer "attempted nation" since I feel it's a bit more positive and communicates the aspiration.