The functionality that you will need to learn about is the Scene Overload options. If full overload was selected when the scene was created, then it won't be possible to add the new parts to the scene. But partial overload should be ok to add new parts. I don't remember all the rules at the moment, but that is the documentation and functions to look up, to solve your situation.
When you created the scene you had product 2 and product 3 selected.
If you would have selected the top level product when you created the scene, then you will see all newly added parts/products in your scene.
If top level is selected when you create a scene, all new parts will come into your scene visible and scenes will then need to be modified to turn products off (hidden) if desired.
I don't know how to modify your existing scene to show complete assembly.
Ricky stated the situation correctly. When you create the scene, select either the top tree node (or select nothing, same result), and create the scene keeping the partial overload option. Then when a new product is inserted, the new product will appear in the scene.
With a little testing, this is what I found:
Selecting specific items in the tree when creating the scene causes only those items to appear in the scene, and no other items can be added.
Selecting the top level in the tree, or selecting nothing, AND selecting Partial Overload when creating the scene, causes new products to appear in the scene automatically.
Selecting Full Overload when creating the scene causes means that no new items can be added to the scene, regardless of whether the tree selection was limited or not.
I tested with your files and you can load them again. Something is different in the behavior of Scene.1 and Scene.8 in this case, and I don't know why it's different, but it behaved as described.
For curiosity, I tried deleting a product and then re-inserting it in the parent product to test what would happen:
Deleting a product from the tree will delete that item from the scene where it was used (full or partial overload, no difference).
Re-inserting the deleted product back into the tree will re-insert it into the scene if the scene is partial overload.
Re-inserting the deleted product back into the tree will not re-insert it into the scene if the scene is full overload.