I want to make sure Slide A is viewed before Slide B

It sounds like you don't want to use Restricted navigation, and you don't want to remove Slide B from the menu. You do want to ensure that everyone views Slide A before they view Slide B. 

Here's one way to do that: 

• Create a T/F variable with the default value of False.
• On Slide A, add a trigger that changes the variable to True when the timeline starts (or after they complete an interaction, if one is required).
• On Slide B, add a trigger to perform an appropriate action when the timeline starts with the condition that the T/F variable = False. The condition ensures the trigger will only run if the user visits Slide B but has not yet viewed Slide A.
  • The options for the trigger's action depends on how you want to handle the situation. Here are some examples:
    • Lightbox Slide A. (Or jump to it, though I think that could be confusing.)
    • Show a layer with a warning that indicates the user must view Slide A first. Include a button to either lightbox or jump to slide A.