When your hot your hot even if the pump drive is not

If you want to increase the shear strength of the shaft, a simple hardening process is probably not going to do it. There will be limited depth to the hardening due to the slower cooling of the interior of the shaft; that slower cooling allows the molecular structure not 'fix' in the harder configuration.

I believe that the hardened tip is not for shaft shear strength but simply to help prevent the corners on the shaft's hex from rounding off in the pumpo socket. If you really want a stronger shaft, then IMHO a different shaft material is called for.