MPPT controller - voltage sensing without separate wires?

Exactly why MPPT controllers are rated at

Input Watts (P=I*E)

Higher voltage lower current same power
Just do not go above the rated input voltage value

and

output Amps

So a 45 amp controller should never be designed with more than a 600 watt array
So long as the voltage developed by the array is less than the controllers max input all is good. As you say anything more is a waste of money the controller limits the output current to it's max rating. Input watts and output current for MPPT