...Um... hes kinda dead.
Then Re-gen the world with the same settings, but instead only gen it to the year after he becomes saviour of the world.
That's not how it works at all. The same settings gives geographically similar world, not an identical one.
Seeds. Random number generators can be made to re-produce the exact same 'random' numbers if given the same seed.
Right. There are four seeds: the general seed, the history seed, the name seed, and the creature seed. If you reuse all of these seeds and keep worldgen settings unchanged, you will be able to have the exact same worldgen.
You'll have to look up your history seeds inside gamelog.txt, which should be inside your DF folder. It should say "*** STARTING NEW GAME ***" then list the four seeds. Go to "Design new world with advanced parameters," press E, and enter all four seeds. If you used the "Create new world" option with all default parameters (medium everything) then you should be fine choosing the MEDIUM ISLAND setting. If not, you'll have to figure out the exact number for site cap, number of civs, etc. that you changed, and enter them—I'm not sure of the exact numbers for the different options.
Now the crucial part is entering the end year of the worldgen so that you end at a date after Monal does the weird hero thing, but before he dies.