I wouldnt have thought that it was a nitrogen overload they were way too yellow for that. You were in soil so im having a hard time thinking that pH was the issue as usually soil is a lot more forgiving with pH.
as a coco coir grower i know that seedlings survive on little to no nutes for the first 10-14 days however if you go beyond that time and there is no nutrient in the substrate you'll start to see signs of N deficiency very quickly which is indicated by yellowing off and droopyness. which your seedlings are showing.
however, there should have been nutes in the soil already (however im not too sure, perhaps you can verify this for me? was that soil mixed with compost? one thing i can tell you is that the soil definitely doesnt look "hot" in my eyes it actually seems quite baron. before plucking those plants i would have attempted to feed with an organic liquid fertilizer (not salt based... oragnic!!!).
just to keep in mind for next time. i feel like this issue was a lot more simple to fix than we were making it out to be...
Also, dont let anyone bust your balls about going oversized with the pot. iv started an autoflower in 50L of coco coir before, literally just popped the seed right in and it did fine. got 8 oz off that bitch too. just make sure the medium is completely and evenly moist before planting then just water modestly untill your plant starts to drink.