Nufar Basil Turning White and Yellow

Two questions regarding our plants developing from Richters "Nufar F1" seeds. I am growing plants in two mediums: Miracle Gro potting soil and a generic "disk" which pops up when watered. I water the seeds and plants exclusively with distilled water.
#1. The seeds grown in Miracle Gro, about 30 percent when the leaves mature to about two inches begin to turn white then yellow from the center out. What may cause this, please?

This certainly sounds like a nutrition problem, but I can't put my finger on exactly what it might be. My first thought is some sort of imbalance that is affecting iron. Iron, of course, is needed for chlorophyll which is what makes the leaves green. If iron is not in its proper form plants can have difficulty getting enough.
I am wondering about your use of distilled water. If you are not adding nutrients and are relying exclusively on the nutrients present in the soil mix, the distilled water will eventually leach the soil of nutrients.
My advice is to fertilizer your plants and see what happens. Use a commercial water soluble 20-20-20 fertilizer at half the recommended rate once a week for a few weeks. You might want to apply fertilizer only to a few plants at first. If you get an immediate greening within two or three weeks then you know that nutrition is part of the problem.

#2. The seeds grown in the pop up "disks" are slow to mature and the leaves appear more fragile, not defined edges, and the green color is very deep. These are less than half the size of the other plants. With these plants I notice that the roots are far longer than the others. I presume the difference in soils to be the reason why there is a difference between the Miracle Gro and the disk plants.

The disks offer very little room for the roots to develop. They may be enriched with nutrients but if the soil volume is small then the roots cannot develop enough mass to support more above ground growth. Once roots grow out of the disks it is time to transplant into soil in larger pots. You plant the disks right into the soil; you only remove the netting holding the disk together (if there is any) if the netting interferes with root development, which in the case of basil is not likely to be the case.

I have about 100 seedlings going, planting 25 at a week apart; all are in pots until we get 50 degree nights – IF EVER!! A few plants, three or four, have developed to about six inches and have what I would describe as "massive leaves". I did nothing different with these plants.

The Nufar variety exhibits some variability in leaf shape and size. The six inch leaves are exceptional though. I don't have an explanation for that.