Order of update execution

I have found the documentation on execution order, which is really helpful. However, what it doesn't say is in which order a number of Update() functions are executed. This can obviously be important as each update can change things that then affect the next update.

They seem to be executed in reverse of the order in which the objects are in the hierarchy pane, or possibly with the newest created object first? Is this true or is it more complex than that?

You can order script execution in the Inspector.

It's undefined. If you need a specific update order, you'll need to manage it yourself (e.g. by having a single 'game manager' object that updates the other game objects in the scene in an order you specify).