Rust spots on leafs curling red stems

Looks like they are suffering from salt build up in the medium. That can cause a locking out of many nutrients. Flushing out your soil with pH adjusted water will certainly stop the problem. I say that because i see nutrient deficiencies like Potassium and Phosphorus. I also see significant leaf tip burn with upwards curling. Go look at my Lemon Kush diary thats almost finished. I believe week 3 of flower you will see what i just described. No deficiencies in my plants like in yours, but i recognized it within 8 hours of feeding that plant and the next day i flushed it. The progressing damage stopped dead in it's tracks.

Looks like you're having issues with phosphorus either a deficiency or maybe even a lockout, everything that you are naming are classic signs the rust spots, leaf curling, and the red/purple stems.

IMO. I would say your pushing the envelope on the feed scale 1.8 ec is high for autos. Ph should be 6,2 to 6.3 in flower as well.