Ahhh, yes... The roots couldn't grow through or into the soaked Promix, because the water was too high. Just needs to be up under the bottom of your net pot and the bubbles will splatter it up, plus since you now have roots(awesome!) they will be in the water a bit soaking up what it needs. With the Promix soaking in there it could have been screwing with the Ph yes, there is lime in the Promix and that raises Ph, it's possible that was the culprit, but not certain.
That sucks more lights are too hot, but if you can get more air exhausted out and more fresh air in, then you could work in a couple more bulbs I think. You've done great at trouble-shooting dude. A lot of 'newer' guys would have seen that slime and all and probably freaked out posting a thousand questions when all they had to do was use a little common sense that hopefully their mama. daddy, g ma, g pa, gave em.

Also, a bit of peroxide to your rez will clear that up with the quickness, and also provides more oxygen, just don't use too much. It's a like a Tablespoon to a Gallon I believe.
We're slow starting, but you're going good considering, the roots coming out is a great sign, my friend. The pic of the seedling is nice and green. Don't worry about the first cotyledons yellowing, once the first true leaves grow after them they are dispensable and always day off.
At this point, since you're in hydro, I think you should add a light dose of veg nutes(green bottle) 1/2 Teaspoon per gallon. There is a little bit of starter nutes in the Promix, but you're only using a handful and the roots are through that now. Basically no nutes in the water, so in hydro you have to provide what they need always. Try a bit and see how she goes.
Glad everything's going pretty good, I've been wondering.