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.