I doubt anyone would've won, like you said one side has nothing to lose the other everything but having nothing can drive someone they were both motivated and while Nala obviously trained her forces she doesn't come off as a military leader Zira does. Zira trained her whole army, she comes off as a cunning strategists. Her plan to infiltrate the pride, the way she organized the ambush and even how she directs her forces. Zira is a soldier of war her forces are not just better trained they have better planning and tactics too. This gives the outsiders the edge but like you said the pride Landers have everything to lose, they have more to fight for. This evens the odds to the point that I think they would've killed each other leaving only a few survivors on both sides.
Zira would've likely beat Simba. While Simba is definitely stronger and more powerful he is likely no where near as trained as Zira or as durable. Zira definitely seems like the type of person who would ignore pain and just tank her opponents attacks. Simba would've likely lost to scar who was not a full on warrior, Zira trained an entire army by herself. Simba isn't that kind of fighter from what we have seen. I think Zira would've killed him especially since Simba wasn't fully healed.
So in conclusion I believe the war would be a massacre leaving very few survivors but I feel Zira would kill Simba before likely being killed by Nala and likely many members of the pride, (I feel Zira would likely take multiple lionesses to bring down) I also think she would die laughing at her killers. So yeah not a pleasant conclusion to the battle and certainly not kid friendly but that's my view of it Zira would finish her main desire but likely get almost everyone else killed in the process. In the end nobody would of won.