I'd cover up the heat ducts in that room, and turn the bathroom exaust on....

Your cooking them - There should be a 5-10 degree difference in day/night cycles. Not a 20+ difference, so there is a small part, another one, is the ideal temps are 72-76 degrees. A good thermometer has an RH reading, this is room humidity (no, it isn't, it's relevant humidity, and no, this isn't edited, I'm high, so this is the shit I think in my head while I'm typing)

If the room is dry, yes, get some moisture in there, but the fact that your budding, you don't want it super moist, my cab is about 20%, I'm just building my new flower cab (DIY guide to come, maybe) and I'm going to keep that at about 20% as well. Could that cause the spots? Well, my BEST GUESS is a combination of the foilar feeding, the low lights (600w should be 18"), the high temps - Because you know if the room temp is 84, the plants closest to the lights are just under 100 - The lack of humidity and perhaps the roots, but if they're not leaking out the bottom hard core, I wouldn't be too concerned about that, but if your runoff is that acidic, I'm sure there are a few things you can do.

Just keep in mind, when you told me to give you my best guess, you told me to leave out nutes

More pics!