Since no one has responded, I will jump in and at least that will bump your post so maybe someone else will see it.
I don't do hydro and don't know anything about it, but I do start tons of plants from seed every year under lights.
To start with you said seedlings with leaf edges curling over, but what you seemed to be showing was stems flopped over. Sometimes lettuce just does that-- it has a little bit of stem before the leaves start. If that stem gets too long, it can bend over. Growing in dirt, I just mound the dirt (actually potting soil) up around it. However the other possibility for seedlings that end up flopped over is damping off. It's a fungal condition that seedlings are vulnerable to in conditions of too much moisture and too little air circulation. You can read more about it by typing damping off into the Search the Forum Keyword Box. But in that case, by the time the plant is lying flat it is a goner. Damping off can be prevented, but not really cured.
But I don't know what the hydro folks do about that. Your set up with the seedlings down in the box and the rockwool staying wet all the time seems like creating perfect conditions for everything to get damped off. So hopefully someone who does hydro will come by and say a little more, or you can look it up else where.
Re the lights. When I am growing seedlings, I keep the lights just 2-3 inches above the seedlings and on 16 hrs a day. I don't know if growing in water instead of soil changes anything re the light requirements.l