The rapid transition to idle is part of the problem, a slowly decreasing idle is more reliable with weak mixtures. In the old days when you could see what car emissions really were (pre cat) at idle speed the co level would be at about 1.5 or 2, raising the speed to 2000 or so made it run much cleaner, below 0.5 or so. Idle needs a richer mixture for stability, which higher speed negates the need for.
Why Fords, particularly fiesta are so bad I don't know, there are some faults that aggravate it a bit, but basically it is a feature rather than a fault. The higher speed is turned on by the ecu, so isn't easily disabled. One way would be to let it warm up and then have a switch in the circuit to the IACV which you could turn off when it was idling nice and steadily, repeat each time from cold would put people off though.