The research they are using is several years out of date. They are going to offer you WBR because its the best they can offer to you right now. It might work, it might not. They can do things like give you alzheimers drugs and try to spare the hippocampus -- you will still likely experience a cognitive deficit.
I don't know where you live but right now new
"Right to Try" laws are being passed across the country and they allow patients like you to try to do something different. Take a look and see if your state shows up, if not it is still probably leveragerable in your favor.
A little bird told me that Genentech knows that IT Herceptin can go after deep brain mets in a way that existing therapies for brain mets do not. They aren't reporting on this as they don't want to get in trouble with the FDA. IT Herceptin is a reach for you without leptomeningeal disease but it might save your life.