As a matter of economics, would a chartered AN124 really have been cheaper than fitting a spare engine pod and flying it (regular service) to the stranded jet's destination, even allowing extra fuel/time spent flying at lower altitudes ?
Well, given that pretty well everything in the airline industry is driven by economics, then clearly yes.
As well as the extra fuel and block time, you also need to take into account the potential loss of payload that results from bolting an extra 6 tons of engine under the wing, as well as the expensive aircraft downtime required to fit and remove the fifth pod.