Nala (and Kovu) were not helping

[Hatari05]

I agree but as I have said before the only character who get's any focus in the first movie is Simba. I look at the second movie as a deconstruction of a lot of the first.

(Note I'm pretty sure you know a lot of this I'm not telling you the story and it's moral but rather I'm explaining how the writers likely came up with the idea of Zira, Kovu and the outsiders and why)

The movie takes the happy ending of the first and completely deconstructs it. Simba did save the Pride Lands and most of the pride stood with him the tension and drama is all there in fact had the outsiders been in the first it probably would've been more dramatic since you now have a conflict beyond simple good and evil but there was no time for that also the movie was about Simba no one else this isn't a bad thing.

It's not they all lived happily ever after anymore instead Simba's journey continues we didn't really see him deal with Scar's betrayal that wouldn't just go away that would swell into overwhelming hatred and dealing with that hatred and moving beyond it is the next step of his journey. So how do you do that how do you show this and create a scenario that would paint a picture of forgiveness and letting go. The hyenas wouldn't work since they were already exiled Simba had nothing to really do with them they also wouldn't have any support for Scar at all anymore and it has to connect to Scar to justify Simba's hatred. This is where the outsiders come in a group who Simba banished just because they were loyal to Scar. This shows that Simba's hatred for Scar is all consuming further shown by his rejection of Kovu who has no connection to what happened at all.

Of course now you need a villain to reflect this story Zira. Zira once again shows the different side of the first movie what if someone actually cared about Scar and her motivation is completely understandable. True she didn't exist but if they worked off what was only in the first movie than you have nothing to use the first was meant to stand alone and left the sequel nothing to work with so when you have nothing you have to just make something. Zira was constructed to show that everyone has loved ones no matter how monstrous they are. Zira was also made to reflect Simba in a lot of ways much like Scar reflected cub Simba in a lot of ways. They both face similar conflicts and make similar choices both are consumed by hatred finally crafting the skeleton of the story one of forgiveness.

My point being the first movie left them nothing to work with they had to make do with something and the idea of the outsiders adds to the overall conflict turning the serious into a almost a cycle of revenge. It gives the series an overall conflict a struggle between Mufasa and Scar and their respective legacies. Is it a retcon? Obviously yes. Does it add to the mythology? In my opinion yes. It really ties the movies together and like I said gives the series an overall conflict it was also the best way to continue Simba's journey.

[SimbasGuard]

In regards to Kovu not jumping after Kiara, if you'll notice Kovu did glace to Simba before Simba jumped after Kiara. I am sure that if Simba had not jumped after Kiara, Kovu would have. If Simba and Kovu had jumped after Kiara together, they would have run the risk of bumping into each other and doing more harm than good.

As to the possible explanations for The Outsiders. This is an interesting FAN THEORY I have herd.

Zira and her Outsiders could have come to The Pride Lands during Scar's reign when The Pride Lands were still healthy.( although not too long before things started to go bad). Scar accepted them into The Pride, thus making Scar a hero to Zira and The Outsiders. It would also serve to inspire Zira's fanatical loyalty to Scar. Also on a personal note I honestly think that Scar and Zira's relationship was...to put it politely, something more than just King and Subject.

As to where The Outsides were during The Battle For Pride Rock This is my FAN THEORY:

I believe that Sarabi wasn't the only lioness leading a hunting party. I think Zira and The Outsiders were out hunting as well, perhaps even going beyond The Pride Lands to hunt in an effort to impress Scar. Because of this, they were conveniently absent during The Battle For Pride Rock.

As for Nuka, Vitani and Kovu, well Disney stating that Nuka and Vitani are not Scars cubs has made FAN THEORIES about them difficult. So I am unable to come up with a solid theory, on who Fathered Vitani, and I have no idea how to explain Kovu unless he is Vitani's littermate and is just the runt of the litter. to explain why Vitani is bigger than him. As for Nuka Zira could have been pregnant with him when she joined Scar's Pride. Nuka was likely only young cub at the time of Simba's return.

[Squeely]

The two probably figured it was too dangerous, and/or figured that Kiara and Simba, being strong lions, could handle themselves. They were likely ready to jump down at a second's notice if need be, but ended up not needing to.

[Amadi_the_Guard]

I think that SimbasGuard's theory pretty much covers how I think.

Vitani isn't all that bigger then Kovu to begin with. She is also scrawnier then he is and a bit more elongated, even as a cub. I personally believe that's her general body structure and nothing more. She might take after Zira on that department.

As for why Kovu didn't jump... well, they probably wanted to show Simba was still the protagonist despite his recent behavior. And what better way to do it then show his willingness to risk his life for Kiara.
I would personally have preferred it if it had been Kovu, or both of them, somehow.