OK so a few things for autos here. I just read through this whole post and I may have missed things, so please forgive me. Long post incoming. Sorry.
-Your VPD you're listing is room VPD. If you measure leaf temperature, it will likely be about 3 degrees cooler ish than ambient and that will put your VPD at about 0.99 kPa. I'd lower my humidity to about 50% if possible. That should raise leaf VPD to 1.2 kPa.
-Your 3 finger leaves indicate a problem. Without knowing your whole grow style and everything, I can only guess and leave it to you to dig through your notes to potentially help future grows. However, I see you mention that they were in 2 gal pots and then that got transplanted into 3.4gal pots. Transplanting an autoflower is highly debatable and can be successful with the right timing. I'd say the most successful transplant would be the earliest transplant. So, of you transplanted within the first couple weeks, then you should be alright. If not, then you may have introduced unnecessary stress. If you look at the pictures from previous, how big was it when you transplanted, and what was the leaf/growth structure like prior to that and then after? That may be the source of your odd growth.
-The odd growth I mentioned is both the 3 leaf leaves as well as the lack of branching. The taller plant you have seems to have mostly a main stalk and bud sites along it, with only 1 other branch that has buds on it. Normally, I'd expect somewhwre around 8 or so side branches off the main stalk, each with bud sites along their nodes.
-The leaves at the bottom WILL start to turn colors and die now. Especially given the distance from the light as compared to the top of the plant. They're not getting as much light and the plant is now focused on growing buds, not leaves. All those damaged leaves will likely only get worse too. So, don't go chasing your tail for the bottom leaves. Just let them die and make sure the overall health of the plant stays in check.
-Magnesium is a mobile nutrient, so it will pull from the lowers if it needs to. If it's a problem in the top part of the plant, it's likely not magnesium. It could be calcium however. Just wanted to mention that since they have different strategies for attack. If it was just magnesium, you could foliar feed Epsom salts and it will give it to the plants, but not cause an imbalance in the soil. Calcium is a bit different.