Mark,

I don't make these types of blades so far. So I can not answer specifically. Off hand I would say the hardness level should be low to medium. Toughness with some springiness would be appropriate in my thinking. You might consider increasing the breadth in that area a little to be safe as well. With the extreme curve of this design, I would think the tendency of it twisting is something to overcome.

This is an opportunity for you to do some testing. This is what the American Bladesmith Society stresses. It's why the JS and MS tests are designed the way they are. We promote this kind of testing especially when the integrity of the piece OR IT'S DESIGN is in question. This way you would be able to quantify your results for future reference.

How much force will it take to bend if the entire blade is hardened and tempered?

That's one of those catch 22 questions.... nearly impossible to even guess.

If it's questionable as to whether the knife will "hold up" under it's intended use, then my suggestion would be to redesign it in such a manner that the "weak" area(s) is/are eliminated.

What hardness should I aim for so it will bend and not break?

Lots of variables go into that question too... IF all things with the steel and heat treat are "correct" (for 1084), it's going to need to be in the low to mid 50s Rc to bend versus breaking. But again, that's assuming EVERYTHING is "spot on" when it comes to the steel and the heat treat.

My best advice would be to redesign the knife to eliminate possible trouble areas/spots. My thought pattern is that IF this is a customer design, then the customer must be willing to accept changes to the design in order for the knife to the best it can be, otherwise I don't make it. If my name is going on it, I'm absolutely not going to build something that I know has a flawed design/possible trouble areas. To do so, is simply taking more of a risk then I am willing to accept.

As I've said many times, nearly everything we do in Bladesmithing/Knifemaking is a "give-n-take" situation.... you usually give up something in one area, in order to gain something in another. In this instance, I would give up that super thin/narrow portion of the handle, to ensure the knife isn't going to fail. If you choose to temper the entire thing back to where it will bend versus breaking, you will sacrifice edge quality. But ultimately, YOU, as the knifemaker must determine just where you'll "give", and just where you'll "take".