Hi, in regards to
#2 question-- Please see the screenshot at the link below, which shows what "advanced variable" field I am talking about. This is not mentioned anywhere in the documentation you provided above.
Looking at your image, you don't need to add the advanced variable any more. The latest plugin version allows you to select the Membership Level which provides the ID as well.
In regards to the following comment.
Can you tell me if this is what has happened?
1- A user was automatically created by paypal,
and
2- I then prompted the same user to come back to the site and create another user? (maybe I should not have done this step? Was it redundant?)
That is what I suspect has happened.