I understand that the time-bound JWT authentication feature of the Google Cloud IoT core is the main problem in maintaining a persistent connection between the Ewon device and the cloud.
As an alternative to this approach, the Google Cloud PubSub service is a data ingestion service provided by Google Cloud which has a one-time JSON key authentication.
As Ewon devices support programming in JAVA, is it possible to run a JAVA publisher client code for Pubsub on the device?
You will most likely need to check what java version the java publisher client supports. The ewon only supports J2SE
You can find more info here: Java | Ewon Developers