I'm planning out an adventure game, and can't figure out what's the right way to implement the behaviour of a level depending on state of story progression.

My single-player game features a huge world where the player has to interact with people in a town at various points in the game. However, depending on story progression, different things would be presented to the player, for e.g. the Guild Leader will change locations from the town square to various locations around the city; Doors would only unlock at certain times of the day after finishing a particular routine; Different cut-screen/trigger events happen only after a particular milestone has been reached.

I naively thought of using a switch{} statement initially to decide what the NPC should say or which he could be found at, and making quest objectives interact-able only after checking a global game_state variable's condition. But I realised I would quickly run into a lot of different game states and switch-cases in order to change the behaviour of an object. That switch statement would also be massively hard to debug, and I guess it might also be hard to use in a level editor.

So I thought, instead of having a single object with multiple states, maybe I should have multiple instances of the same object, with a single state. That way, if I use something like a level editor, I can put an instance of the NPC at all the different locations he could ever appear at, and also an instance for each conversation state he has. But that means there'll be a lot of inactive, invisible game objects floating around the level, which might be trouble for memory, or simply hard to see in a level editor, i don't know.

Or simply, make an identical, but separate level for each game state. This feels the cleanest and bug-free way to do things, but it feels like massive manual work making sure each version of the level is really identical to each other.

All my methods feel so inefficient, so to recap my question, is there a better or standardised way to implement behaviour of a level depending on state of story progression?

PS: I don't have a level editor yet - thinking of using something like JME SDK or making my own.