Hi Hetal
Apologies for the late reply as I have been away.
To answer your questions:
Ideally you should use the Integration Engine for PO since PI will eventually be decommisioned.
4 & 5. Like in my previous reply. It does not matter, because you can configure which interface points to which IS_URL by configuring the interface in SXMSIF if you are using proxy.
6. If you are using IDocs, then need to change the partner profile to use an IDoc port that points to the new PO system. For outbound proxy, the RFC destination used for the above IS_URL will need to point to the PO's SOAP adapter - please refer to the link I provided, it's all there.
Rgds
Eng Swee