When revisiting a slide, doesn't work like I'd expect it to

If you're going to show the seekbar but want to ensure the user views the entire timeline, set it to "Allow drag after completion."

That setting prevents the user from dragging the seekbar until the full timeline has been completed. Here's the User Guide article. 

Unfortunately, even though the user can drag the seekbar after they've viewed the full timeline, the built-in replay button doesn't work. To replay the slide, the user would have to drag the seekbar back to the beginning and then click play. 

An alternative is to add your own Replay button on the slide. That could be hidden until the timeline ends.

The attached file demonstrates both options on the same slide. The Slide Properties are set to "Resume saved state."

FYI, according to the User Guide, this is how Storyline "automatically decides" what to do when revisiting:

• If the slide has only simple objects—text, images, audio, video, etc—but no interactivity, Storyline 360 resets the slide to the beginning of its timeline.
• If the slide contains interactive elements, such as buttons or other objects that have a visited/selected state, Storyline 360 resumes the slide where it left off previously. 

Here's what I recommend for the “When revisiting” setting in Slide Properties:

• Keep "Automatically decide" for graded-question slides. Storyline will know whether to reset the slide (when the user needs to answer the question) or resume it (when reviewing).
• For other slides, change the setting to ensure it does what you want.