Good morning,
From reviewing your use case, the way that OSB is being used does seem to add redundant functionality that the CA API Gateway is already providing and could be customized to do. You could approach it one of two ways it that you keep the REST front end of the Gateway and convert the requests to be supplied to Siebel or import in the WSDL (as mentioned) into the CA API gateway so that it is directly available as SOAP to your clients.
Sincerely,
Stephen Hughes
Broadcom Support