First Grow, don't die on me!

I think the biggest problem here was, as you suspected, overwatering. The fact that they STILL have green on them says to ME that they CAN be saved - at least it's worth the try. Give the stems a little bit of support so the leaves are not lying on the soil and are as upright as you can GENTLY make them... this might be with a piece of paper folded into a V - something that is not going to poke into the soil at all and disturb any of the roots. Then also lower your lights to about 11" and let them be... do NOT water them again until the soil is really dry. With luck, they WILL survive - and weed is a pretty resilient plant so the odds are good. Seedlings need only a little bit of moisture to grow... when they first pop from the soil, they don't have much of a root system and it's VERY easy at this point to drown them... the soil should just be sprayed a bit and always always always left to dry out before giving them another drink. Good luck... you CAN do it!