Howdy....I read the entire thread. When I first looked at the pictures I was a bit puzzled with that leaf curl....leaf curl can occur because of different things...but its the way that the leaves curled that really caught my attention. Usually with that type of leaf curl its because the plant is trying to disipate water but cant. Why cant it...that is the question at this point. I think you had a combination of problems going on at the same time and I think that a few might of touched on it. I know that you just transplanted but can you tell me if that fist potting mix had a NPK ratio on it? If so, what was it?
It does appear that it was running hot...not the temp..the soil and at the stage you are in...that will KO a seedling quite quickly. When you transplanted, did you notice that the soil might have been a bit to wet...a little soggy?? Over watering will produce that type of leaf curl but you had something else goin on as well which looked like the soil was hot...a little nute burn. When popping seeds, try to use a soil that is light and no nutes added to it....some touched on it here...a good mix will feed your plant for several weeks without you having to do any kind of nute feeding....fox farm happy frog is a good example of that. I dont know if thats available to you where you are at but something similar will wk
PS....I know you are using Apple cider vinigar for PH down....regular distilled white vinigar will wk just fine. You dont need to spend money on high dollar PH up and down. Trust me...I've been growing for years...it wks just fine. If you want confirmation of that..just go to
www.Mandalaseeds.com and check out thier grow tips. These guys have been in the biz for years....if its good enough for them...its good enough for me and you and.............