How to properly connect to game engines.

[sDIMMaX]

It would be more accurate to ask the question - how to most optimally connect - keep the handles in objects and get the position of the object directly from the side of the game engine, or make a layer that holds a pair of handle-game object and transmits it to the side of the game engine from the side of physics?
As far as I understand, the layer is more optimal, but where and when it is more clever to receive information about poses, because it is most appropriate to transfer a new pose only to those who moved, the only thing I saw was IntegrateVelocity (or is there a ready list of moved bodies?) How appropriate is it to transmit directly in this callback? since I don't want to interfere with batching and C# JIT (the transfer simple data can call external functions (C++))
I think make a loop and have List<bodyIndex> movedBodies and have List<GameObject>[bodyIndex] bodyGoPairs
i.e -> foreach (int index in movedBodies) -> bodyGoPairs[index].GlobalPosition = simulation.Bodies.ActiveSet.Poses[index]

And question, IntegrateVelocity can call several times per one body in one timestep?

[sDIMMaX]

And second question - how property save state? can i save only pose and velocity and on load -> simulation.Bodies.Add(new BodyDescription.CreateDynamic( ... state ....) ...) or need save all simulation data? (i.e. exist cases when in games save-load-save-load- you can get outofbounds)