Stainless Steel is a pain to work with, I feel your pain! I've broken a few lathe tool bits.
I don't have a lot to add, but one thing that comes to mind since your main problem is the parting off. Could you set up a simple cold saw setup where a secondary saw blade driven by an electric motor is used for parting off? Similar to a toolpost grinding setup, but instead of grinding, you'd be sawing. Then the Hex corners wouldn't be a problem, and the lathe could rotate it super slow to get a nice even cut all the way around.