I'll be honest, the first time I saw this theory I said "No." And I STILL say no, but up until now I never found a reason. Well a friend of mind named "LadyKakata" who can be found on dA(warning, she doesn't like SP so prepare to bristle) expressed all my thoughts exactly. She's a bit of a swearer so I cleaned up the dialouge a bit
"It is a debate if it causes sides to stop and think about their points. To be honest, there wil be points where I have one opinion and you have another, and we'll either agree, disagree, or think about our respective points in ways that we didn't think before until the debate
the 'Kopa was killed by Zira' thing really does stick in my craw though, it makes absolutely no sense. Scar got away with killing Mufasa in the original without Queen Sarabi tearing him to shreds because she didn't know he did it. Simba was the only one who could have told her the truth, as well as Zazu if Zazu had witnessed the events (which he didn't; he was knocked out by Scar) and the hyenas if the hyenas suddenly decided to betray Scar. However, Simba ran away and the hyenas interests run with getting Scar into power, ergo, Sarabi does not know the truth until Simba forces it out of Scar as an adult.
If Zira killed Kopa, she'd need to deal with the entire Pride and Pridelands as well as Simba. A small list of reasons that this theory does not work follows thus;
Zira is not mentioned in SNA
No Outlanders are mentioned in SNA. Or TLK for that matter
Kopa is not mentioned in SP
Zazu only mentions 'murderous outsiders' without mentioning what it is that they have done, and neither Simba and the Pridelanders nor Zira and the Outlanders bother to clairify what it is that is supposed to have happened. Zazu implies they've killed someone, but Zira implies it's because she's pissed at Simba doing something ('when I think of what that brute did'), i.e. killing Scar. Really, neither says what it is and perhaps this is very deliberate in order for the fandom to make up the reason so the freakin scriptwriters don't have to. 'Mystery' is easy to try and apply to 'plothole' you jerks!
If Zira killed Kopa, she'd need to deal with Simba. He's already lost his father and in essence his childhood, he doesn't need another death so soon and to someone as precious as a first-born child. He's clearly got a temper and a vengence streak, look at how he told Scar to his face that he didn't deserve to live and clearly doesn't listen to Scars pleas for mercy. He wouldn't be afraid to kill his sons murderer, no matter how happy he is being back in the Pridelands or how good at heart he is
Nala is a warrioress and has spent basically her childhood and adolecense in a Hellhole with the threat of forced marriage/rape over her head as well as being terrorised by hyenas with little to no food available. She doesn't take Simba's BS, or anyones BS, and the idea that she'd let her childs killer walk free is utterly insane. She has a greater reason to murder Zira, as Kopa is her first child and maternal instinct is not a mistress that backs down and mellows out when it comes to the death of a child. Zira would be a smear of organic jam on the walls of Pride Rock
The Lionesses would all be protective of the first new Royal child into the Pride since Simba. They would be cautious of Zira to start with and hearing that she murdered Kopa and in essence the guarentee of a golden future would send them into a frenzy.
The Pridelands is just getting back into shape and wouldn't take lightly the death of the golden Prince to favour the 'chosen' successor of the male who MESSED UP in the first place. As much as the Pridelands is a passive character in the first film, there is still clearly management karma in place and the animals DO have an interest in the monarchy that manages them (they came to Simba's ceremony). They would have a problem with everything being screwed up again and would possibly intervene if the consequences of not doing so were severe enough (they can move onto other lands though, so it's a case of weighing moving again against killing a stupid lioness)
Timelines, as mentioned before
If Zira had managed to kill Kopa and make it look like an accident, that does not explain why NALA is so relaxed in SP NOR why Simba doesn't give Kiara a full protection squad to make sure the same thing does not happen again. The idea that Kopas death resulted in Simba's irritating behaviour, yet it doesn't make it strong nor change Nala's perception of cub safety, is incredibly stupid
If Zira killed him and made it look like an accident, then why is she banished? To say she killed him but was banished for another reason is like picking two full, rich fruits and tossing one casually into the trash for you to pick out and dust whenever it suits you. Stick with your choice or don't freaking pick two fruits.
tl;dr Zira killing Kopa is stupid >:C
I really hope I'm not completely steamrollering you though; it's rare to find articulate and thoughtful members of the TLK community that don't buy into the crap of TLK2 (*IchigoSeppun and ~werebereus are two)"-LadyKakata
Wise words even I could not argue with.