Can you add water as the eggs are pipping to increase humidity? One pipped on day 20 so we removed the vent plugs per checklist, but put them back after about 6 hours. I'm afraid we might have reduced the humidity too much. I do not have a wet bulb thermometer in the incubator. The chick has pecked a small hole through the membrane, but has not made any more progress.

Yes,

You can add more water if needed. I'm not sure whether that will help the one chick you are concerned about, however.