Kindly find my answer in below lines & let me know if it helps:
Ques: This means I have to use ACL, don't I?
Ans: Yes you have to use ACL to achieve your objective.
Ques: This means, not registered people can't access any data. Probably this could be solved by using a dummy user for those people I think...
Ans: No, you don't need to insert the dummy user in ACL app. If user is not registered with us then we enter owner as anonymous user. So he/she will access the docs in that collection, if you have given the public read access to the doc.
Ques: But I can't change the ACL setting for an existing app, can I?
Ans: Yes, you can't change the App settings from Non Acl to ACL.
Ques: And as far as I saw it, I cannot give access to ALL other users, can I?
Ans: Yes you can provide public read access on your app data, this will enable Read access to all your app users.
Let us know if you have more question for us.