Under-watering? I water all my plants equally and ...

When they are small in there seedling stage u need to be waiting just on the outside of leaves tip end to end. There small roots is only as wide as leaves. Keep watering like this and expand radius as seedling grows. Do this for the first 2 weeks of life till she has a good root system. Then water the whole container. How u need to water to 10 to 20% runoff when u start using nutrients. Runoff gets rid of salt build up and old nutrients and replaces it with fresh. Also only feed water water then feed again. Do not feed every watering. As for when to water just lift pot when it's light it's time to water. Last yes you can save them they just need some liquid seeweed helps recover from heat stress and prevent. Just little tlc bounce right back.

To me your soil was under watered and too hot. Too hot because it burnt the plants and under watered because of the droopy leaves ( could also be a damping off). How much water ? Ir is not really that the problem, 1st you need to water around the plant and not only at the basis to help the root system to thrive and search for nutrients, 2nd you don't need a water amount to give but a general soil moisture to reach : the soil need to be moist but not over watered. The best way to know how to water is to lift the pot when soil is dey and remember the weight then water and lift the pot and remember the weight : when the pot will be light weight you'll need to water again

Each plant is unique genetically and will grow and feed at different rates. Yes keep them they will pull through if you make a few changes. So it looks like some nutrient burn possibly the soil was a bit too hot for the seedings or sometimes over watering in the beginning can cause the issue also. Another potential cause could be that I your light may be too close for the seedlings 600 watts at 18 inch is too close if it's a true 600 watts I would back it off to 30 inches or what ever the manufacturer recommends for distances. Remember directly under the center of the light is the most intense. If the light is too intense it can stunt the young seedlings. As for watering I like to use a turkey baster to control how much and where I place my water and nutrients for seedlings and young plants. Your larger plant I would give it about 50 to 75 ml of moisture/nutrients every 2nd or 3rd day as needed. in an area of 6 inches in circumference around the young plant. The smaller seedling do the same thing but use 25-50 ml of water every third or forth day as needed. I usually use a chop stick or my finger to stir up the top 1 inch of soil around the seedling /young plant to see if it is dry or damp. With Autos usually you can start feeding 1/8 or 1/4 strength nutrients after 10-14 days if there are no nutrients already in the soil. Hope this helps👍