You probably did not need to add soil, but this would not be caused by new soil. It sounds like you did everything right with one serious error: you did it in early summer instead of early fall. New grass plants do not have strong enough roots to survive summer heat.

You watered as needed. What does that mean? With the new seed, how frequently did you water and for how long?

How are you watering it now since it has become more or less established? How often and for how long?

Very much a disease issue and likely Pythium. Check it out on the web and become familiar with it. As Dchall pointed out, spring plantings can be killers. Those young plants are so susceptible to disease and they're heading right into summer. I'd also question your use of the Sun and Shade mixes. Primarily perennial rye, KBlue, fine fescue and not always the best varieties. With the size area you have and water considerations, I'd suggest you consider tall fescue. The new turf-types have just as much shade tolerance as the fine fescues and are far better in full sun. Once you get them established, their extensive deep root system will likely find you some water savings and 1-2 fertilizations a year will get them by. Yes, they're coarser textured grasses, but on a large lawn they can look great. If you decide to go that route this fall, TF should be planted 2-3 weeks earlier than the Sun & Shade mixes. They'll pop just as quickly but shut down sooner in the fall as the temps fall and day lengths shorten. 6-8 lbs per 1000 max for those areas front and center and for the 'back 40', 6 lbs will work. Good luck!