Can you restrict access to a learning plan within a community? 

Yes. You can restrict access to a learning plan within a community, by assigning it to a specific audience. For information on how to assign a learning plan to an audience, see the Editing visibility section of the Visibility tab. 

What happens to a user's progress score when content is added to/removed from a learning plan?

• If content is removed from a learning plan after it has been published, the user's progress score will increase.
• If content is added to the learning plan, the user's progress score will decrease (as the user has not yet viewed the newly added content).