Good Evening Phenoman,
Gosh, it's really hard to even see an issue with your plants; they look that good, but I do see what you're referring to.
1. It most certainly could be a hot spot; sounds like you're mitigating that issue by moving the duct work.
2. I would highly doubt that the plant has issues in only one part of its root system which is causing localized problems on the plant. I would think an issue in the root system is going to affect the entire plant, not just parts of it.
3. In terms of the LED, I have no experience using them; I use CFL's for veg and HPS for flower. It's quite possible that the over lap of the HPS and LED is causing an issue, considering the LED's have a history of bleaching out leaves and so forth. I've often wondered what the LED's affects are when you consider they are known for bleaching out leaves and flowers when placed too close...and this bleaching doesn't appear to be heat related as much as it is spectrum related. Never the less, perhaps Pennywise or some of the other folks here could comment further to this end.
4. It doesn't appear your nutrition is off...the plants, save these few leaves, look pretty darn good and green. (A little leaf tip burn is nothing to worry about.)
If I had to put money on it, I would say perhaps you did have a hot spot...but I mean over all, they aren't looking bad...actually they look pretty darn good. I know it's easy for me to say it, considering they aren't my plants, but I would try to be patient, watch them and don't make any quick moves to correct them. A day or two, in the life cycle of a plant that will run, let's say, 120+ days over all, isn't much...about 1%...don't be too quick to react. Take a deep breath...watch them...and if you feel they're going south, go with your gut. You didn't make it this far by chance my friend; you know what you're doing.
Try to enjoy your evening...