hey ilikebike,
I don't think plants suffer overdoses of light. They just won't produce more sugars then normal.
Bummer most seedlings didn't do well. They looked good when sprouted. Must have been an overdose most likely. In my opinion you cant really kill them with too much water when in seedling stage, they usually just grow slow or stop growing. But giving seedlings too much water for too long time, then it probably can be very difficult for the plants.
I don't understand why you didn't transplant them immediately, those containers you are using are made to be submerged into the soil entirely, the roots grow through the container. When i used those, "carton" containers i transplanted immediately them after they sprouted (recommended in the store where i got them). Can't go wrong really.
If you are going to germinate seeds in pre fertilized soil, then you won't have to feed them for the first week most likely. After that, start testing, like: add 1/4 or 1/2 of the recommended doses on the package or grow schedule. If they take that well, then 1 or 2 weeks later raise to full doses.
When transplanting to bigger pots, again think about the amount of nutrients in the soil. So be careful the first week after transplanting with adding nutrients.
What i often do when not being sure about the soil. First i water the soil before transplanting, adding full doses of nutes, then checking my plants age and size to decide wether i´ll be giving them half or full doses.
Another most important factor is when adding nutes and wondering how much, is the amount of light the plants will get AND the temperature, day and night, where day temperature being most important.
Rules of photosynthesis prerequisites: Light, Water, CO2, Optimal temperature. For indoors also wind and air refreshing is important ofcourse, since windless days are usually rare. air in soil must be refreshed also, adding animals, like worms and crawling creatures do that outdoors a lot. A bit of wind blowing across the bottom of room, or wind coming from underneath helps a lot, and get pots with big air holes ( you can make them bigger or add more if you like, i made them slightly bigger, since the had a crappy finish from production ) or even better pots
Good rule for starting, 1) get the room conditions OK or better (seems you got that done), doesn't have to be perfect, 2) get good soil with imo not too many nutrients but enough for a healthy start, 3) and be gentle on the fertilizing, first time you'd better be off using a bit less then you'll probably will be using later. You'll have to learn detecting deficiencies and later on when knowing what deficiencies are, you can see how excesses are being shown.
Seeing the results of changing nutes in for soil can take a couple of days, i say 3 or 4 days at least before knowing the results, if they change looks and behavior right the day after adding nutrients then you have stacked the excesses of nutrients, after that still adding more nutrients or the same dosage isn't a good plan. Sometimes you'll see a quick positive reaction like lower leaves turning greenish again, usually a good sign, but don't raise again, give them couple of days to show the result. I made that mistake often in latter days of my experience.
The only exception i encountered was, when plants where in accelerated growing stage, they take a lot of grow nutrients in that stage, so when adding nutrients the plants started growing faster, but were also pulling a lot of N out of lower leaves after 2 or 3 days when started adding nutes> That's an exception i think. As long as the plant doesn't suffer severe lack of nutrients, then it wont harm them that much yellowing some lower leaves. I think it's also quick nutrients for them, quicker than retrieving from soil, so sometimes they really need that.
Good luck, i would definitely transplant the survivor, submerge the whole container into a big pot asap. The can't grow much roots in there and half a cup of soil wont buffer much water or nutrients.