All Objects On Screencast Showing up At Once While Editing

In the timeline pane, to the left of the objects, there is an eye icon. Clicking on it will make that object invisible. When you are through editing, clicking the eye icon at the top will first turn them all invisible, and a second click will turn them visible again.

FWIW, for me, best practice would be to split that screencast into 10 or 12 sections, and put one on each slide. Start the video when the timeline starts, and unless you need to pause for learner interaction, jump to the next slide when the video ends. It avoids this problem, gives you so much more flexibility, and keeps the timeline a manageable length. All of these lead to shortened development time (you can cut it in a few minutes with MS photo, or the free version of Lightworks), and the learner never knows or cares how many slides they see; all they see is the content.