Hello Stewart,
Firstly, i have not worked on HL7 integration, but have worked on EDI integration using AS2 adapter.
I have got your flow, which would be one synchronous and two asynchronous scenarios.
For synchronous, you need to generate a proxy which triggers the data to PI. And the structure for your proxy interface would be the one provided by HL7. Please use the same XSD for both sender and receiver.
To connect to HL7 via AS2, you require Basis help to open the port, setup HTTPs connection, import SSL certificates etc. Certificates also need to be exchanged with AS2. Both parties, SAP PI and HL7 must have their certificates CA signed.
Make sure the above mentioned setup is done properly. Once done, you will have no problem in connecting to HL7.
And once the request reaches HL7 and acknowledgement is sent to SAP PI, how it is being handled? You are sending it to same proxy? If yes, then your sender interface also must be a synchronous one.
If the acknowledgement is being handled in a different way, other than proxy then you need to setup a Sync-Async bridge to handle acknowledgements.
Similarly the other two acknowledgements needs to be handled.
Regards,
Nitin