Resolution:
The Workflow can be invoked with and without Integration Objects.

i) In case you have selected 'No' for Integration Objects in the Configuration tab of the Adapter Service, as you have done for the RequestResponseService, the data will have to be passed in the Application Context of the invoking(BW/IM) activity. The adapter will create a propertyset called 'Tibco Message' and will send to the WorkFlow which will read the hierarchy.

ii) In case you want to configure the Integration Object schema and have selected 'Yes' for Integration Objects in the Configuration tab of the Adapter Service. In this case the data will have to be sent in the BusinessDocument of the invoking(BW/IM) activity. The adapter will read the data and create a propertyset called 'SiebelMessage' to forward to the Workflow. The workflow should read SiebelMessage instead of TibcoMessage in this case.

Note: The change required on WF to invoke the Siebel WF using IO from adapter is to make the WF input to read the SiebelMesage instead of TibcoMessage propertyset tag.

How do I invoke a workflow with/without Integration object?