I dont know if its overwatered because well im not the one watering it. But it sure looks like its getting too much water, the soil looks moist! A cup like that in 50% humidity and 70f-80# should take a couple days to dry out under cfl's. Can you post pics of your drainage holes? Sometimes depending on soil and drainage, the first 1" of dryness can mean nothing. Since its just a seedling, let it dry out and weigh it. It will give a general idea of dry weight since it really wont pack on much weight for a week or two. The trick here is to get roots without under watering. It is extremely sensitive to heat,water and nutes at this point. In those solp cups, you can get by with 20watts of cfl for a week just to help your high temps. 40watts next week and so on, hopefully by the time you get back up to 80watts you have your ventilation figured out. By that time it will be alot harder to kill the plant because the window of stress is alot bigger vs a seedling.