Hi Tony.
Before starting the upgrade you can refer the SAP Upgrade guide from SAP SMP
1) what is the correct path to upgrade from NW 7.11 to NW PI 7.4? Do we need to upgrade to NW 7.3 EHP1 first and then upgrade to NW 7.4
You can upgrade current system to NW 7.4 before that check this SAP note for PI connected NW systems (like SAP ECC, BI, Portal) Check the PDF
1388258 - Version Interoperability between SAP Business Suite and SAP NetWeaver Systems
2) How to advice the customer on NW PO versus NW PI? What is the difference, if any?
Refer the SCN link Installation Options for Process Integration and Orchestration Use Cases
3) Can we upgrade from PI 7.11 to NW 7.4 single stack ? Will it cover all functionality we have in current NW 7.11 dual stack system?
If the current system is having dual stack, you can upgrade the existing system to NW 7.4,
Regards
SS