To be honest, I'd go for two batteries in this case, I feel like undervolting the axe motor that far would end up being quite a hindrance as far as power output goes. Yes it does mean two links, but stick a small 24v battery in there and I doubt it would add too much in the way of complication, and you'd get an awful lot of gain - could even adjust your ratio a little too to reduce the strain on the motor and get a little more torque out of the axe itself (excellent for self righting)

As far as the design goes though, I really like that - it seems rather solid and certainly something different! What materials are you planning to use for the build?