Cost effective and easy way to leave the current setup (ECU grounds controller @4.5K) and have an override switch in the system, Is to place a diode inline with the ECU switched ground (0v) and the switch after the diode (see Pic). The diode will let the 0v from the ECU through but will block the 0v from the switch going back into the ECU.
(Warning! Diodes are directional, placed in this circuit the wrong way around will harm nothing but it will mean that the switch when on will send 0v to the ECU and when the switch is off the 4.5K rpm ECU output will not pass through the diode)