Using examples from the Microsoft CDM github and exported entities in a "live" production environment i have had no issues attaching a model.json to create dataflows of several connected Entities. ...
Sorry, this did not work. The json is just the schema file for the CDM data. It doesnt really hold any data by itself.
Since data from CDM is stored in headerless .csv files you need to load data through the CDM manifest/schema file. This in turn points to the csv files and applies headers, formats the datatypes and build relationships (if such are defined).
Exporting several entities to a CDM folder residing inside a datamodel (as in D365FO Entity Store) it works fine using dataflows and attaching the model.json.
When working with just a manifest / schema json nothing seems to have been adapted to it, ingestion wise.