Before I say anything: Yes, I know Kopa is not canon. Yes, I know Kiara is officially the firstborn in the movie universe. Yes, that is how Disney intends it to be. Now here's what I actually have to say.
I always just assumed it was Kiara, and I wouldn't have any problem accepting that, except...all the inconsistencies.
The cubs do look a little different; I guess if you stretched it you could probably say they were the same cub, but that's debatable. But the part that really bugs me is their presentations. Fluffy/Kopa's is during the day, high noon or something like that, but Kiara's is at dawn, and there's no way you can possibly say those presentations are the same. Doesn't necessarily mean this is a different cub; it's just an annoying inconsistency Disney made. But I don't like accepting "It's a Disney movie, roll with it" as an explanation, so it forces me to think that maybe "Fluffy" actually is Kopa. Canon or not, it makes sense. As for what actually happened to Kopa if he did exist, I don't know. Maybe he was killed, maybe he left for some reason. With all the facts adding up like they do, I have to reasonably think a non-canon character is canon, because it's the only logical explanation I can find.
That said, I go back and forth all the time with my theories. In one fanfic I'll have Kopa as a 100% real character, in the next one he'll never have existed at all. So I guess I can be persuaded either way.