Maybe i'm totally wrong here, but what is exactly the "difference" between difference and the result of a subtraction? I'm pretty sure the result of a subtraction is called difference.

To this point...
CV1 = 3V, CV2 = 5V, Subtraction = -2V, invert that to get my 2V
I think i would just switch the two voltages, if that is possible in your use case.
CV1 = 5V, CV2 = 3V, Subtraction = 2V

I mean it's always good to know about alternatives, but Maths does all of this without a problem. I wouldn't even bother to use an inverter. A Full-Wave Rectifier makes more sense imho. MI Kinks has one.