Can a pending state be cancelled? (currently we have boom, civil war and unrest are pending)
Not exactly, but some can be effectively avoided if you have a higher priority state.
States have priorities:
Lowest: Boom, Bust
Low: Outbreak, Famine, Civil Unrest, Lockdown
High: Expansion, Retreat, Investment
Highest: War, Civil War, Election
If you can get two states of different priorities to go active on the same day, the states appear to be checked from lowest to highest, so the higher priority state removes the lower one milliseconds after it goes active, and it disappears from the Pending list and you don't get the effects. It looks and acts like it was cancelled, in this case.
If the higher priority state goes active first, then the lower priority state will hang around in Pending until the higher priority is finished.
If the lower priority state goes active first, then when the higher priority state goes active, it will overrule it. This can lead to lower priority states disappearing quicker than their documented minimum length.
So, in your case, the civil war is unavoidable as it is a highest priority state. When it goes live, depending on the exact timings, it may take the boom and/or the unrest with it. It's also possible - if you're currently in state None - that the Civil Unrest will cancel the pending Boom, and then itself be overruled by the civil war in a day or two.
