Earlier quoted context omitted.
What function is the Rails component serving? Is it just a REST interface or is it also acting as a separate HTML interface?
You can do both, for example, you can use rails for authentication with traditional views and then use ember for the main single page app. I think this is how square does it. Also you should look into active model serializers for integrating rails with ember. I think it's yehuda Katz' preferred method for Jason serialization from rails to ember.
That is true. AMS + Ember-data = 👍 (I'm now maintaining AMS, but Yehuda and Jose Valim originally wrote it for this purpose.)