As for the changes in predictions when you change MN:
first, the time change will only be in minutes and the fuel in hundreds of kilos, so using ETA and EFOB for calculations is not accurate.
Anyway:
If your CI is 40 kg/min means that it is worth to accelerate and arrive one minute earlier only if the extra fuel burnt incurred is not more than 40 kg.
this is very difficult to see accurately, specially in short legs. In long flights, you can see that the econ mach is good for your CI, because increasing mmach in 0.01 increases your fuel burnt in more than the CI value per each minute saved.
If you had exact fuel and time figures in the predictions, to the kg and to the second, you could increase your mach until the increased fuel burn was higher than the CI. That MN would be your ECON MACH.