Nation replacements won't occur during the game. Sweden is an optional nation that can be activated or deactivated in the settings
You'd have to check Periods.c and ships_init.c but I don't know anything more specific because I don't play NH that much. If the Surprise is a quest-only ship it should be only permanently for sale from Vanderdecken, but never appear in random encounters.
I don't think that you can easily change the date in a file during a playthrough, you can only skip time through the console. But then again, the world won't dynamically change