I don't think so. And as canon I now mean the sequel movie The Lion King 2: Simba's Pride because it's the main pinagmulan for those theories.

It's true, they can't ipakita every little detail about crucial plot parts in a film. But what they can do is refer to everything crucial. Like, in this case, if there had been another cub before Kiara, they could've and would've sinabi so in the film.

In example, they could've had Simba and Nala briefly discuss it instead of how much Kiara reminds them of Simba. Or, combined the two topics.

Nala: "She's just like you when you were young."
Simba could've replied "Exactly! Remember how our son Kopa was just like me and he disappeared/got killed sa pamamagitan ng the outlanders! And remember all those danger we got ourselves into!"

I mean, Kiara was far away sa pamamagitan ng that point--the discussion was free to be about anything in the world--and adding that one line would've taken less than five segundos of their minuete limit. And instead of having Nala be careless, they could've had her walk away worried/sorrowful indtead of walking away carelessly.

But no, instead they have him only remark about their personal childhood dangers and Nala poke him about it and carelessly state that Kiara will be fine. Because there was never a son at all, not to mention a murdered son.

ALSO...

...Kiara's protection and supervision is left in the hands of a tiny meerkat and a warthog. Creatures that are easily killed sa pamamagitan ng a lion, not to mention numerous lions. If someone really wanted to kill the princess, they would madala a warthog's stink for a little while. Especially as it would stop stinking after it's dead or at least couldn't follow around. As in if Kiara's parents had a child who'd disappeared or been murdered, they would have their segundo child much, much better supervised and protected. Especially as she is eveidently just like Simba who himself used to manage ditching unreliable babysitter.

...Zira is an evil psycho who takes pleasure in blood spilling and war. Yet she doesn' brag about how she succeeded in killing the prince and revenge Scar's death sa pamamagitan ng it. Instead she whines and complains about how unjust the exile was and how she is about to get her revenge for Scar. Obviously she hadn't done anything to deserve the exile and the very least anything that gave her pleasure.

...Nala is completely carefree and light-hearted about Kiara. Doesn't seem like a loving mother who has lots a child.

Simba never succeeded to put Mufasa's death completely behind him judging sa pamamagitan ng how he had nightmares and tears in his eyes about his father's death even years and years after the event.

What makes you think he'd succeed any better with a loss of an only son?

Especially as there was always something to remind him of it: Kiara. And Kovu, as he was Zira's son. Leading me to how the nightmare was only about Mufasa and Scar and Kovu becoming Scar. If there was anything about a lost/dead son in Simba's inner life, they would've had it in that dream sequence.

And in the end of TLK he put his own guilt behind, not Mufasa's death. Scar; "I wouldn't want to be responsibly for a death of a family member. Wouln't you agree, Simba?" Simba: "It's not gonna work, Scar. I've put it behind me." The guilt. Not the death.

If Simba only thought Zira killed Kopa, she might as well have. It would affect Simba's inner life the same or even worse actually, as Simba could not be absolutely sure and it would cause him worry and doubt about if he's child is at peace in death or being tortured/slowly dying somehwere.

A tragic loss like that would not go away just because "it's over". Having your child murdered... And especially if the child's fate was uncertain, it naturally wouldn't be over. And again, the film makes no references whatsoever to any other trauma than Mufasa's death.

Sometimes I wonder if people who support those Kopa theories, have ever had a close family member disappear or die? Those loved ones are with you every waking moment and especially in your dreams. The pain of the loss, the worry if it's just disappearence, and the joy of good memorie. You don't get over it so completely that you stop dreaming and talking about them. Even if you don't talk about how they died or disappeared, you talk about how they lived. You don't just forget about them just because you have other/new family members. Not even with the rest of the family there for you. On the contrary those who are still with you, inspire you to remember the Nawawala one.

Even Simba says it in the film: "Even those who are gone, are with us as we go on." If Kopa was one of "those who are gone", he would've been mentioned sa pamamagitan ng his family or even shown in Simba's dream.

You might say that it's a kids' film that doesn't necessarely make the characters react realistically. But they made Simba react realistically to Mufasa's death and Kovu and all, I see no reason why they wouldn't in a Nawawala child case too. Because the point and the theme is exactly the same.

Then there's also the outside appearence problem.
Kopa and Kovu appear the same age, while in the books Kopa is still with his family and eager to become king. The cub at the end of TLK is at least three months younger than Kovu because lion cubs don't pop out of thin air, their mothers remain pregnant for a time.

Thus if Fluffy was Kopa and grown up to the age he is in the books which is at least nine months of growth, then died or disappeared, then Kiara made, carried, born, grown up some time... Kovu in the film would need to look much older than he does because he'd be over two years old at the movie's time. There is no way lack of good pagkain can make a lion lack that much in growth--especially as Kovu apprently did grow up to a normal adult size in the end, so they apparently didn't lack that much pagkain all the time.

Anyway, Kopa and Kovu would appear exactly the same age which is why Kopa's existence as a Nawawala cub in SP's story would be impossible because Kovu was born when Scar was still alive and Kiara in the film is such an old cub while she isn't in the books at all. And the Outlanders are not in the books in any way while they should be as they were there in the film's backstory all along.

Hence, if the film makers wanted an older brother of Kiara to exist, they'd just simply written a story where's a place for him to exist and then included him in. That's the whole point of story writing: you write a story so that everyone you want to include in it, has a place in it. They didn't include Kopa in any way and wrote the story so that his logic existence is impossible--thus they didn't want him to exist.

And as they used the cub from TLK as baby Kiara in their official movie trailer, it seals that Kopa never existed in their story. And yes, even if the trailer was made sa pamamagitan ng the marketting people, The Walt Disney Company does see this trailer as official as it has the word "Disney's" on it and it was released on an official Disney VHS.

The only way to logically connect Kopa into the movie universe is to first ignore the fact of the official trailer and then have him born after the sequel's events and write Kiara and Kovu out of Pridelands. But even so it would be partially a logic fail with: the books don't explain why all the sudden Kiara and Kovu aren't the future reyna and king but Kopa is, and why wasn't Kiara shown at the end of the first film even though the sequel made clear that she was to be the susunod ruler after Simba.

There is no completely logical way to connect Kopa into the movie universe because officially he was never meant to be part of it.