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 fonte for those theories.
It's true, they can't show 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 said 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...