Can the namespace preferences and program preferences be set via REST API calls? If yes, what is the syntax for it?
Generally in Cloud Data Fusion, when we intend to perform the action on GCP side, like create/delete/restart etc. instance, it's feasible to use domestic Google Cloud API, giving the opportunity to interact with a service endpoint via JSON/HTTP calls interface as described in Google Cloud API design document.
Dedicated to Data Fusion you can follow the Cloud Data Fusion REST API reference document, nicely explaining the methods for composing REST API HTTP calls to manage Data Fusion instances, moreover every method description from the documentation contains Google API Explorer sub-panel, to get handy experience building JSON request on a live data.
Said above, I assume your initial question is related more to CDAP REST API, as it includes the methods for pure CDAP instance metadata/namespaces/application configuration.
From the user perspective your workflow might be the following: