Conditional Actions that would make something appear on a future slide in the lesson

When using a simple action with that button, the dropdown lists with objects will only show the objects on the active slide. However when using an advanced/shared action the dropdown list for the Show command will show all objects in the project. I don't understand why you look for a conditional action? A standard action could do the trick. Be careful to use a good labeling system to be able to recognise the image that you want to make visible. With a similar work flow you can hide that image again.

What is not possible is to change the state of a multiple state object that is on another slide, unless that object has been timed for the rest of the project and also shows up on the active slide.

You can eliminate the step of storing a value in a user variable. While you cannot show an object on another slide directly from the On Success section of the Actions tab in the Properties panel, you can use the Show command to show an object on another slide from within an Advanced Action.