Try testing the runoff (pH). Also, I recently ran into the similar problem where I was dialing in my water at 6.2-6.5 for soil and was locking out the calcium and magnesium. I had to resort to applying my cal/mag via a foliar spray until I could water again. For soil you want a pH of 6.5-6.8 in order for your plants to absorb those elements. Hope this helps.
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Checked your soil mix again, I see a lack of carbohydrates.
Soil life needs carbs.
Fresh banana peels and any other fruit and veg scraps will definitely help.
Compost / worm tower!
Also way too much vermiculite and perlite.
It’s a waste of volume that could be used for organic matter, you don’t need it in a no-till bed unless your climate is extemely humid all the time, the roots of the previous grows provide the same function and provide nutrients on top of that.
Anyway, it’s in there now, best you can do is add as much and as diverse organic matter as possible.
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Update:
The plants don’t look much happier but the flowers seem to be developing. Each of the plants has its own rather nice smell now.
That makes sense and seems to be happening, yes. Pruning the affected lower leaves just causes others to get the same way.
Soil or air humidity? It’s been difficult to keep air humidity low, although with the external dehumidifier it’s been hovering around 55% now.
Agree 
I need to figure out pH testing before trying this again.
My testing was not precise but the result was clearly on the acidic side rather than alkaline. I tried a simple pH up with the last watering by adding a tablespoon of baking soda in the water. It’s only been 2 days and I can’t tell any difference yet.
Not at all. The sides always feel dry, I’ve only felt it wet at the very bottom where it sits in a tray.
I never watered so much as to get runoff.
Yup, I’ve been adding kitchen scraps (in moderation) and there’s been a good amount of pruned companion plants left on the soil. I never see the worms (as expected, I guess) but there’s plenty of fungal activity (mycelium and the occasional tiny mushroom).
(btw, love Geoff Lawton’s stuff!)
Makes sense and definitely something I’ll keep in mind if and when I try this again. I stuck with what seemed to be common guidelines for this first attempt…
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Well if your pot is only a bit wet on the very bottom I think you can rule out too wet soil. If your keeping your RH in the air at 55% your good there so long as your temps are in the low 70’s F.
The leaf tips curling down could also be heat or too strong light, or a combination of both, I did some tests on a plant in my seedling cabinet, I am using to do a full grow temporarily. I deliberately put a spare plant close to the light, bumped up the heat and the results looked very similar to what your suffering with, only more exaggerated, but it’s hard to say for sure.
Plant before test.
Plant at testing height.
You can see how the plant has dropped all its leaves to reduce surface area to the light. It took about a week to bounce back after lowering it again to its original position.
As I said, overall I think it’s mainly PH issues you have, but I don’t think that would affect your cover crop as well, I have never had a problem with clover or alfalfa from any issues that my weed plants were suffering with.
Baking soda is not a good PH adjuster because of the salt in it, but small amounts occasionally should not be a problem in the size of your pot you have.
It may be the strains you have are very fussy, I have had a couple of strains that would get very unhappy no matter what I did with them.
Its a very perplexing problem you have 
If you have spare ingredients to make more soil up, I would start building it now so it has time to break down before using it in your next grow. I don’t think I would re use what you have in that pot without some re conditioning.
You can by pass PH problems with foliar feeds just before lights out, I would start with cal mag, and then a balanced tea of NPK the following day, if you have the ingredients to make it handy. Dont spray the buds just the leaves.
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Thank you for the test, it’s (ahem) illuminating and I appreciate it.
It would make sense that heat and light are one of the issues here. Until last week, air temperature has been in the high 80s and the light has been at max intensity:
I didn’t get to make the thermal camera work yet so I don’t know what the leaf temperatures are (I calculated VPD based on air temp).
The light has an IR switch that I had turned on when it was looking like flowering began (i.e. not long before problems started happening). The dehumidifier was also started soon after, you can see the slight bump in the temperatures on the graph.
Last week I turned off the IR lights and dialed down the light a bit, which brought the temperatures down.
This has become academic for now because I’ve ran into a new problem: the room with the cabinet now very recognizably smells of weed 
I’m not sure whether the carbon filter is not working as well as I thought it would or (more likely) the very leaky enclosure is causing the issue despite the constant negative pressure inside (the fan is running all the time now). Either way, this is a serious issue because we sometime have visitors in there and they absolutely can’t know.
I’ll be chopping the plants today. I’ll take a picture just before – I don’t think they are ready but the Purplematic might be close enough.
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I find the hotter the room or tent is the more the plants smell.
I remember one time when I first started growing indoors I had no temp or humidity controls, the exhaust fan was either on or off and I manually turned it on in the morning at lights on. The smell of the plants was not too bad and localised to the room the tent was in at 82f with the fan running full on.
I had a 1000w HPS light dialled down to 750w still to hot for a 4x4 ft tent.
Went out early one morning and forgot to turn the fan on, came back after the light had been on for 6 hours and as the wife got out of the car 30 ft from the house she said, what’s that smell lol.
Man you could smell that weed through the whole house and outside for 3 days. The temp in the tent was 105f for 6 hours and they wreaked lol.
Chopped everything down two days ago. This is what it looked before:
… and after:
Hoping something will be useful after drying, but not holding my breath. Nevertheless, it has been an enjoyable and educational experience. Main lessons:
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Need to observe more and fiddle less. I thought I was doing well about this, but there’s room for improvement.
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A better pH tester and a stronger magnifier would be useful.
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This community is even more friendly and helpful than it seemed at a first glance. Thank you!
If and when I try this again, it’ll be in a tent. I’ll need to figure out whether to keep it in the same space (mostly out of sight, climate controlled, prone to the same smell issue) or put it in the garage instead (perhaps less likely to cause a smell issue but more conspicuous and can get hot).
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Live and learn!
You still got something!
Next run will be better, no doubt. 
Thanks for sharing the journey. 