Word of caution: This question is related to question How to create ContentType with FieldLinks using REST but is more specific as we are looking to find a solution until (if?) MS brings the API Change to make it possible
Afterwards we proceeded to create the columns, also using REST (reference doumentation MSDN site column reference :
Next we created the Content type using JSOM, as it is not possible to create Content types using REST on the web Level (at least we got Bad Request). The same REST-attempts using http:////_api/web/lists(listid)/contenttypes (for single lists) worked, but we wanted to do the association at CT-Level, not at list-Level.
What did NOT work at all were the FieldLinks. According to Microsoft MSDN Documentation for REST-FieldLink Objects it is not possible to do this via REST.
Thanks in advance!