What about - instead of the cooling causing the dieback directly - you simply get belts of devastation from the Force 10 hurricanes spawned by the sudden cooling?

The first El Mega Siblings event would have been a BIG surprise for settlers, unless someone had taken the time to predict future orbital conjunctions very precisely. Subsequent ones would be braced for appropriately. But the exact path wouldn't be predictable in advance, due to many complicating factors (such as the rotational position of the planet as it enters the cooling zone, which would change which land masses are involved). So visitors (e.g. PCs) using historical data for the last Mega Siblings Season are likely to be surprised.