humidity looks ok, and the temp shouldnt be a huge problem, it could be a root issue with the old compost, you see the new leaf coming through do they look a darker green than the leaves that are dying, if they do then you may be ok, it looks like the dying leaves have dried out to much and probably wont get better, but the growth in the middle looks a bit greener than the rest, im sure other growers will come over as soon as they get online, i asked a few so should see them soon, im hoping the new growth and leaves at the very top look better than the others and may pull it through, but id say if it does end up dying then start again using some compost other than mg, it really does cause a lot of problems and kills many young plants, but you have transplanted it so it will take a day to recover from this as some roots may have been damaged so might not be enough root mass to fully support the whole plant.
1 thing you could try is cut off the single set of fan leaves, this will then stop the roots sending fluid to them lowest leaves and will direct it further up the plants, its the same thing we do when we take a cutting, we remove most of the leaves and cut some in half as their are not enough roots to get enough liquid into the plant, if we didnt do this it would just wilt and die, so for now just take off teh single proper leaves, the next set after the seed leaves, pinch these off at the stem, this may be a case of root damage and the plant is trying to get to much out of the roots and starving the whole plant, their should be enough nutrients in the new compost to support the plant for at least a week or 2, so i dont think the problem will be nutrients at this stage, now you have sorted the fan and got the temps down a bit by raising the light, so it may pick up now, if it was just lack of water then you would of noticed a huge difference all ready, so we need to start thinking of something else, so for now all i can suggest is pull them bottom leaves off without damaging the stem, just give them a pinch with your nail at the stem and pull them off, this may then force the other leaves to start growing a bit better,
at the minute id say it was a root issue, it wasnt over watered as you had not watered since sat, but depending how deep the roots where they may have needed water, but you have watered it now so we know its getting water, it may take some time for the roots to recover from the transplant, so pull them leaves and keep checking it every few hours, dont worry to much about the yellow leaves as these may fall off and die at some point, its the new growth you need to check, look at the colour of that and if it looks a darker green than the dying leaves then it should be ok, its going to take time if its going to recover, but im hoping its all good now and you have managed to solve an issues their may have been, other growers will come along when they see my msgs,