Account linking against an App, not Org

Hi, I have two questions regarding account linking.

Question 1:
From what I can see, one can only manage account linking for a user with an entire org, not against an app. Is there possible to do this with just one of my apps, not the entire Org?

Question 2:
I have now implemented account linking as it says in the docs. I have a problem: The user IDs from step 3 and step 5 are identical. I would expect step 3 to give me the OrgScopedID (as one only has specified the org id in the URI), as it now does. However, I would expect step 5 to give me the user’s scoped ID for the app one has specified in the access_token from step 4. It dosent give me this, but rather the same ID as I got from step 3. How can I get an app scoped user ID after initiating account linking?