Can placeables can be removed/destroyed/created during gameplay and how would that affect the walkmesh?
The walkmesh can *never* be modified during gameplay. Whatever comes out of the toolset is what you’ve got. But… There are two kinds of obstructions to movement in NWN2. There are static obstructions and there are dynamic obstructions. If a placeable is marked as a static obstruction, then it gets baked into the walkmesh and becomes a permanent movement blocker. If you place a chair in the middle of the room and flag it as “static”, then no creature will ever be able to move into the space the chair occupies. Even if that placeable is destroyed, it will still leave an unwalkable hole in the walkmesh. If that same chair is instead a “dynamic” object then it will not be baked into the walkmesh and its state as a movement blocker can be controlled by script or whatever. From a performance point of view, static objects are preferable by a wide, wide margin. So the rule of thumb is to use static objects when you can, and dynamic objects only when you have to.
