Two, surely? The previous one still being used and the new one being written.

(Note that this is how most rendering artifacts were fixed long ago - the on screen and the off screen buffers were swapped, so nobody would "see" in progress scenes)

1 copy + the original = 2 instances in memory.

That's fine if actors are not doing much (e.g. just pathfinding and shooting), but is likely to fall apart long before scripting becomes a thing, since there are a lot of tasks where different actors do want to mutate the same object.

This is AI, having multiple actors plan to gather the same resource etc isn’t necessarily a flaw. It does however result in meaningful game design decisions getting tided up in how the game engine is designed.

This is a great example. It's probably pretty insignificant from a players perspective in most contexts. And you could most certainly design some acceleration structures to specifically handle AI convergence specific to your use case.

Can you share an example where this model falls apart?