What you have is called blossom end rot. In Texas it is caused by the plant being unable to take up potasium, other places it may have a different cause but the cure is the same. Sprinkle about a cup of Epsom Salts around the base of the plant and water it in well. This will not help the fruit already affected but it will stop any more fruits of being affected, especially the younger ones. Overwatering will cause cracks and splits but not blossom end rot.
Next year when you plant your tomatos do it when you plant and again about 2/3 of the way through the season on your indeterminate plants. You probably won't have to do it twice on your determinates
Where are you located?
Unless you can show me pics of earwigs actually on and eating a healthy tomato it I find that very hard to believe. Earwigs eat dead organic material and other insects. They are mostly carnivorous. If an earwig was eating on a tomato with blossom end rot it sure didn't cause the rot. Maybe it is a lack of calcium uptake or maybe it is a lack of potasium uptake, the results are the same. Epsom salts fixes the problem and so might calcium carbonate. I know Epsom salts works in alkaline soils. It is possible in acidic soils that calcium carbonate is the cure. But the problem sure as hell ain't earwigs. They might be a symptom but not the cause
In my experience low nighttime temps causes the blossoms to fall prematurely and also leads to leaf curl but that's all. Use the Epsom salts now. It will not help the fruit already affected but will stop any more from getting it.
You would add it at time of planting and to be sure you are safe use both ES and calcium as neither will be harmfull. The 1 cup of ES will end up around the root system anyway so the volume of soil is not a major factor as ES does not burn, but a tomato needs a big container like a 5 gallon bucket. Like Bees said, the soil in your container is a major concern. If it has no calcium it must be added and in containers it can be easily flushed away.