What is causing the yellow leaves

Well, first of all, autoflowers don't like to be transplanted at all... but since you've done this, they needed time to recover ... yellowing leaves is a pretty natural occurrence when seedlings try to recover from being transplanted. Had you germinated these in the larger pot, you would, about now ,be wanting to slowly introduce nutes to her - but only at 1/4-1/2 strength (ppm about 180-200). You gave them 350ppm the day after they were transplanted and their roots were not capable of using that much at that point. Cotyledons turning yellow and dying off is normal...(going through your daily list of actions taken here) .. as long as you've got some real leaves going, their function is over. Droopy leaves on Jan 5 were trying to tell you the plant was still having a hard time recovering from the transplant - it had only been 5 days. The drooping could also have been telling you they either had too much water or not enough... I can't really tell from your pics which one it was... but I have to say your soil looks pretty dense and is probably holding on to a lot of water... next time, mix a lot of perlite into your soil - weed roots need a lot of oxygen and this helps deliver it to them. Feeding them at 1500ppm was not a wise decision here although I understand what you were thinking... "they need something, let's give them some food and they'll grow strong" ... but it doesn't work that way... and 1500ppm is a massive amount that wouldn't even be tolerated by a full grown autoflower. Sadly, what you're going to have to do is to get rid of those nutes in her soil ... but I can't reasonably advise you to flush the soil because that would pretty much guarantee her death... so no more nutes... plain, pH'd water for the next week at least (letting her dry out completely before giving her more)... which brings me to another point - 5.8pH is too low for a soil grow... you've got to raise that to 6.0-6.5 at least. I wouldn't give up on this grow yet.. as long as the top leaves are looking good (and they are), you've got a chance. But no more nutes! And make sure you're watering AROUND the plant, not right up next to it... you don't want that stem to damp off. Finally, after a week of plain water, if she survives, she's probably going to be wanting to go into pre-flower which will require bloom nutes but she'll also need a bit of grow nutes... I don't know what nute line you're using but use only about 1/8 strength grow nutes and 1/2 strength bloom nutes ... ppm should be about 300... If there are no adverse reactions at that point, that ppm can be raised slightly... but never allow the ppm - during her entire life - to get above 600... Good luck... sorry to be such a bearer of bad tidings but autoflowers are terribly finicky and unfortunately, you've made a couple of bad choices... no matter, with luck, she'll turn around and surprise you! I'm pulling for you!