I don't think switching back to veg will decrease thc, but it will take a long time. people say that when you reveg you can expect to set your schedule back about 4 weeks.
in regards to seeds, yes, you can plant them too shallow. if they are planted too shallow they run the risk of drying out. I like to plant my seeds pretty deep, about a quarter to a half inch deep and I keep them on a windowsill where the environment isn't too harsh. I water over the seed everyday with about an ounce of water until they break earth, then I start my normal watering routine (watering entire cup and letting it almost dry out).
leaving a germinating seed in a grow room near a fan or too close to a light is a sure way to dry out and kill a seed.