I think that you have a multi-fold problem with the big plant (s?).
Check the runoff of the soil for pH problems (would be low by the looks of things) and flush to rid it of toxic salt buildup from all the troubleshooting you had already done. Then next time it dries out, transplant it to a larger pot. That pot is too small to support a healthy plant of that size. Go REAL easy on any foliar sprays next time. When in doubt, mix them WEAK!!!