1. I don't have my PCI Design manual in front of me, but I think you can take pretty close to fpu if the strain compatibility analysis shows that you develop that strain in the cable.
2. This technically correct, but at the level of strain I'm talking about with two cables in a 20' deep member, whether rectangular or triangular, the steel strain will be well above where it needs to be. This is a non-issue in my mind.
3. Point taken.
4. Point taken here, too, but again, I started this thread with the sole purpose of finding out if you can develop the breaking strength of a prestressing cable that isn't prestressed. I definitely appreciate and welcome the other comments and thoughts, but they're outside the direct scope of the question.