Now I don’t really know much about advanced science
But I have some ideas...
Is it not possible that iron oxides have enough oxygen in them to fuel a large amount or the combustion of the coal?
So if you burn coal in the presence of metal oxide ores you will get pure-ish metal, some depletion of atmospheric oxygen, but something far more useful (carbon dioxide rich atmosphere. I’m sure colonists will have their own air supplies which would be fairly well contained within the bases, so what happens to the atmospheric composition outside the bases is not so important... but warming the planet up by releasing greenhouse gases could be far more useful? Especially if ships start sending back rare metals to earth and returning with small amounts oxygen required for the closed loop bases?
Once the atmosphere warms up, and is more conducive to life, maybe we could put bacteria there that could release co2 and o2 themselves by breaking down certain minerals? Or even light fires to melt the co2 ice caps?
One last thought, is it not possible to mechanically or chemically draw oxygen out of an atmosphere using some or another energy intensive process? Carbon monoxide could maybe be easily generated on mars and can be useful As a gas.