I'm not sure I understand the entirety of your situation but I will attempt to answer some of your questions anyway.
I agree the former designer used a round key because of lower stress concentrations in the shaft and disk. The strength of the key in maintaining the integrity of the joint is...
I have to admit not knowing the details of the hydraulic hybrid drive system. But I have worked on electric hybrid drive systems for buses. The pluses are generally that you can recover the energy normally lost in braking, you can run the engine at it's most efficient and/or cleanest points...
