Stuff like unconditional surrender wouldn't have made any difference. Niflheim was not in just to take the Crystal, their intention from the beginning was to take the Crystal, kill the Lucis line and wreck Insomnia to the ground. At no point or choice would Insomnia have been spared.
The movie makes it clear several times: Lucis was going to lose the war. There was no way in which the could have won. The King was weak, the Wall was not lasting and he wasn't able to summon the Knights of the Round.
There is one constant, no matter what: Insomnia falls, and thousands will die.
The variable is: Do we try to keep the last heir to the throne alive so he can use the power of the Lucian kings to defeat the world-consuming Empire and rebuild Lucis? Or do we allow him to die, thus dooming whatever salvageable remains of Insomnia to Imperial rule?
You make it seem as if it was either Noctis' life, or Insomnia's existence, and that Regis had to sacrifice Insomnia to save Noctis' life. This is not so. Insomnia's fall was going to take place regardless Noctis lived, or died. The FUTURE of Insomnia (And the world) actually does depend on whether Noctis survives.