Hi Nabi,
From the SAP Gateway side I guess (haven't done such use cases by myself) all should work.
If the UI5 side supports the file upload via POST as you mentioned, the offline store would work too.
Best regards,
Ken
PS. As you are already aware, my H2G should help you to get the idea about navigation links for creating associated entities during $batch request. How To Implement OData $batch Processing with Content ID in SAP Gateway