First off, I want to let you know that Facebook does not recommend using Instagram for general user authentication. They want you to implement Facebook's authentication instead. From Facebook:
Instagram Basic Display is not an authentication solution. Data returned by the API cannot be used to authenticate your app users or log them into your app. If you need an authentication solution we recommend using Facebook Login instead.
While they still allow Instagram authentication, they could remove that functionality at anytime or reject your application for using it. The Instagram provider plugin that is included in our JFBConnect package is only included for our users that have had it implemented for years (before Facebook changed their recommendations), but we don't recommend using it for general users.
As to your specific questions:
1. With Facebook authentication, the user's email address will be imported automatically. It's been a while since we've used AcyMailing, but they (to my knowledge) use Joomla user plugins to fetch the user's email address and have plugin hooks that detect a new user registration. JFBConnect makes the proper calls to notify any user plugins about a new user so they *should* be added to Acy as expected.
In the case they are not for some reason, JFBConnect creates 100% standard Joomla users under the hood. We just extend that user with connections to their social networks. So, functionality in Acy to 'sync' users definitely will work if necessary to periodically pull in any new user accounts created through JFBConnect if it's not done automatically.
2. If you'd only like to use the Facebook social login for Instagram, disable our other JFBConnect provider plugins in the 'jfbconnect' group.
3. Here are the specific instructions for integrating social login with Easysocial, particularly if you need to set up your own profile with custom information to pull into the user's profile. www.sourcecoast.com/jfbconnect/docs/thir...ation-for-easysocial