Analog Throttle Control - Mechanical Engine

Firstly Can the Polarity of Two Outputs be Reversed in order to Change direction of a Motor ? If not i can used a single digital output via two relays to reverse the polarity !

I would like to fit a Potentiometer controlled throttle to my machine , I would have a 5V input for the Throttle Request and the Linear actuator im going to use has a feedback potentiometer installed to give me actual Throttle Position , Can some help with the code to do this , When the Throttle is increased in the cab the Mc42 Needs to output 12V till it reaches the actual on the linear Actuator - The linear actuator needs to always equal what is requested in the Cab -The Actuator requires reverse polarity to Retract

Can someone please help with the PID loop / code to make this happen ?

Help would really be appreciated as im new to this - Many Thanks