That's actually an interesting point you make about Fluffy and Kopa. I actually talked about them some in one of the
posts on my TLK fan theory blog. I think that "Fluffy" was just a placeholder name because they didn't actually expect to make a second movie so Kopa and Fluffy could technically be the same cub.
As far as Kion's situation, I tend to agree with you. Simba seemed kind of rough on him in Return of the Roar (and also seemed to favor Kiara by not only spending more time with her but thanking her for telling him that Kion had recruited his friends for the Guard) but they seem to have a pretty good relationship overall. I compared Taka's experiences with Kion's in another
blog post and I think Kion has things way better than Taka did.