Thanks for inviting.
For the 1st query, your query target is entities in DEPhysicians? or DEPatientVisits? If want return entities in DEPhysicians, but inline expand navigation property DEPatientVisits, and want filter entities in DEPatientVisits with StimulatorGourpId. Try:
localhost:33396/FalconCPDataService.svc/DEPhysicians?$format=json&$expand=DEPatientVisits($filter=StimulatorGourpId eq guid'27F321EDAB-14C6-4FF2-9485-00ABD176EBC4')
For the 2nd query, Try:
localhost:33396/FalconCPDataService.svc/DEPhysicians?$format=json&$select=FullName&$expand=DEPatientVisits($select=Diagnosis,VisitDate;$filter=VisitID eq guid'F321EDAB-14C6-4FF2-9485-00ABD176EBC4')
All above query is following OData Protocol V4. $select and $filter can be inline $expand. But, I am not sure if it is implemented in odata4j. In sample service, the following are the similar query you can refer
http://odatae2etest.azurewebsites.net/javatest/DefaultService/Orders?$expand=OrderDetails($select=OrderID;$filter=ProductID%20eq%206)