Yes, spree is just for the store. We are using its authentication system which is based on devise. I tried to make it pluggable in dashboard so you can add whatever auth system you want. You will just need to tell it which User class to use, and provide a brewbit_current_user helper function to return it. For the dummy app, it would be easiest to just return a fake user object and not do any auth at all.
We were using unicorn at one point, but have now switched to puma. Still need to update the docs.
The brewbit.com app is meant to be just a shell application that pulls in all functionality in via mounted engines. In the engines folder, there is another gem called spree_brewbit which pulls in dashboard and adapts it to the spree environment.