Let's work backwards on the answer to your questions.  First, 15 hp is too large for the mixer.  The 63 mm diameter shaft can only tolerate the loads of a 3 hp motor at 10 rpm.  Any more torque on your anchor impeller will bend the shaft.  At the same time, 10 rpm does not require much power to turn the impellers.  Your anchor impeller could handle up to about 100,000 cp viscosity at 10 rpm.  Your mixture of coconut oil and water should have a much lower viscosity, probably only 2,000 cp, or less at 70 degrees C.  A marine impeller at 10 rpm will not contribute to mixing and will not require any additional power, or so little you don't have to worry about it.