You seem to be missing the whole point. Half of your statements are completely incorrect.
1. You are able to use the Agilent controller and LabVIEW together. You use the VISA functions to communicate. These replace the GPIB functions.
2. If you can find an IVI driver, you can use it unmodified. IVI is based on VISA and you should alwasy use VISA.
3. That is correct but who cares. You should be using the VISA functions.
4. That is correct as well but again, who cares about using the GPIB functions when you have VISA.
Neither NI, Agilent, and probably the majority of users, suggest that you use GPIB functions. Learn to use VISA and your life will be much easier.
Use VISA instead of GPIB!
Use VISA instead of GPIB!
Use VISA instead of GPIB!
Have I made myself clear? Use VISA instead of GPIB!