It's a small one, but one of the main themes in the movie is the importance of brothers and brotherhood. In the original Prince of Persia game, Jordan Mechner (who helped write the film) used photos of his own brother to create the Prince's sprite. Jordan's brother brought the Prince to life. Neat, huh?
Related to the lack of sand zombies: If Dastan could extract their Sands of Time, he'd probably just abuse the Dagger of Time at every pivotal plot point. Perhaps that's why they weren't in the movie?
Nizam displayed surprisingly competent swordwork immediately following his Villainous Breakdown, the reason he was such a pushover during the film's climax was because his hands had been burnt in that timeline.
What difference is there between Nazim and Dastan holding the dagger in the Sandglass. They both wound up back in time like they wanted. What if the present WAS destroyed but a little side effect of time travel is it undoes that... Still BILLIONS of lives are on his head, if only for a moment. And that was the gods' plan. Now their waiting to see if Dastan felt any remorse for killing all life on Earth after his second chance. Heck maybe Sand Demon would pursue him in a sequel who are basically the doomed masses of the alternate future.
Uh hello! Dastan was trying to STOP Nizam from killing billions of people by messing with the past. Which I'd like to point out, HE DID! I'd also like to point out that Dastan wasn't interested in using the power of the Sandglass, he was just trying to stop Nizam from doing so.
Indeed, throughout the film we see Dastan only uses the dagger in order to save lives, unlike Nizam who intends to use it to take them. Likewise, once Dastan learns of the history of the dagger and the consequences of what would happen if someone abused it's power, he immediately decides to relinquish it back to Tamina!
Unfortunate Implications: The reason that the Persian royalty have many wives isn't because the guys are chauvinist. It's because fatalities for mother and child during birth were extremely common in those days. They needed multiple wives, not as trophies, but to insure the kingdom would always have an heir. Grim, ain't it?