Each of your answers are below:
1) Can it be pre populated from CSV?
Currently there isn't a tool that can write data into Firestore from a CSV, you'll likely need to write some code yourself to change a CSV row into a document and write it to Firestore.
2) Can it be used from AppEngine Golang?
We have a Firestore Golang SDK, which gives you a great client to interact with via Golang.
See this page for more information: https://firebase.google.com/docs/firestore/quickstart
Note there is this warning from that page:
Cloud Firestore and App Engine: You can't use both Cloud Firestore and Cloud Datastore in the same project, which might affect apps using App Engine. Try using Cloud Firestore with a different project.
3) Can it be used as web service backend with 2)
Yes! You can directly write to Cloud Firestore from your webapp frontend or through a backend.
Hope that helps,
-Tyler