I am one of the few who sides with Simba in TLK2, then
I don't think his actions towards the Outsiders, discriminatory as they were, were completely unjustified. They are Scar's followers, and Scar ruined Simba's life. Thanks to Scar, Simba lost his home, contact with his family and friends, his innocence, experienced a highly traumatic event and was made to bear a false guilt for his entire adolescence and young adulthood, which (seeing as we are attributing human characteristics to these characters) are turbulent enough times on their own without you being made to think you caused your father's death.
And I think the over-protectiveness is NOT because of Kopa (he doesn't officially exist), but rather once again coming back to my "Simba has anxiety/PTSD issues" theory. Notice how he tells Kiara "stay on the path I marked for you" - he knows the trouble he got himself and Nala into from evading chaperones and straying from the path ("Don't you remember the dangers we put ourselves in?!"). Even sending Pumbaa and Timon after her all the time, the way I see it he isn't trying to avoid a "Kopa" repeat, he's trying to avoid a repeat of a situation similar to what happened in his own childhood. Blind faith that Scar was trustworthy was what caused the disaster, so Simba isn't leaving anything up to chance on his turn at the bat.
Though that being said I don't think Simba should have forgiven the Outsiders so quickly (even after Kiara's whole "we are one" monologue and Zira's death).
-What differences do you see?
-Well, they are openly supporting the lion that killed my father, convinced me it was my fault, destroyed the Pride Lands and tried to kill me several times. They also tried to kill me like a few days ago and spent the last hour or so trying to kill me again and the rest of my Pride. Also your boyfriend originally joined the pride so that he could kill me. So, yeah, plenty of differences. Not to mention that their leader is about to try to kill me again and will nearly kill you in the process.
Hm, not sure my teenage daughter and a few sunbeams after a rainstorm would be enough to convince me that it was time "to put the past behind us"