It looks like the brown spot problem is all in the older leaves, which lends more support to magnesium def. I have a feeling the problem with your limp leaves is related to overwatering. You may have started a bit of root rot in the bottom of those buckets.
All that said, I feel pretty good about sticking with my first recommendations:
- Confirm the ph meter you have is well calibrated and working
- Let the soil dry out before watering again (checking through the holes in the bottom of the pots)
- Start watering with distilled water for a couple weeks, adding a bit of cal-mag or epsom salts (staying conservative with dosage)
- Long term, find a better way to manage water and ph (no more vinegar, consider either RO filtering, bottled water, or perhaps phosphoric acid to bring down the ph)
I don't know what size your plants were when you moved them to 5 gal. pots. It may have been a bit too early, allowing water to stagnate in the bottom. Next time be sure to water lightly when you first transplant to the bigger pot.