I think we can safely rule out the light as a cause of the droop.
They do indeed look like classic over or under watering, but I've been following along and you've been doing just fine so far. So we can rule that out also. your plants have also reached the maturity to where there shouldn't be an issue with over-watering, even though they are in soil.
The only time that I have had a plant Wilt so severely and stick with it over a few days it was actually a root rot problem in deep water culture.
I'm perplexed, if a plant is under attack by root aphids they can Wilt that bad. However, you usually have one plant that they prefer over others and they just go after it like crazy. So I'm not so sure it's a root aphid problem. Do you see any Flyers around?
Let's see how long it takes those pots to get light again. That will be diagnostic a little bit in itself. If there is something going on with the roots it will take the pots longer to get light.
if they do take longer to get light then the solution would probably be to allow them to wilt severely on the dry side and just give them plain water followed by whatever kind of root stimulant you may have say an hour or so later, basically you would give the plant the opportunity to drink up the water it needed before introducing the nutrient.
lastly, I do think we have an infirmary around here still. And you can get some pretty good help in there.
oh yeah, one last thing if you suddenly had a big change in humidity to the high side that can also cause a big wilt.