They are not even canon pairing! They come from different canons. Kopa is only in the books and vitani is only in the movies.

None happened officially. They aren't even officially together. You can make up if they have cubs or not, or if Kopa was killed by Zira (popular explanation of her exile). I perosnally don't beleive they are together, because of Kiara and Kovu being together, but you can use your imagination and create whatever you want
:D

Well, my answer's going to be simple. What ever you want. The sky is the limit. There are no given facts about this whatsoever.

I think that everyone thought Kopa was dead but really he lived and married Vitani when he grew up and took over the Pridelands from Kovu.

I think that Zira killed Kopa and since he was very young when it happened, he didn't have time to mste Vitani and heve cubs, so I hope that this helps!

You should ask person who made that fan fiction. As far as books and movies go, Kopa and Vitani never met and so could not have nay relationship and cubs. But there are some fan fictions which puts them together.

Also character you linked is not official, it is fan made character. Made by FlightofChaos. And is not connected to any movies or books, beyond fan made stuff. Also character is protected and as such can not be used in your fan fictions/pictures/... without permission of FlightofChaos.

Also Kopa is one of unexplained TLK stuff. Since he is only in books and not in movie. What happened to Kopa is more stuff of fan fiction again. Disney explained nothing about it. So there are people who say Kopa is real and state various reasons why he is not mentioned in TLK2. And some who say Kopa never existed in official TLK universe. So here you are on open grounds to make up your own mind on what you want to believe in.