Yes, definitely more than a water problem, but the water problem can be the initial, that encourages further problems. So always wise to tackle the water problem first. Let that lady dry out for a couple of days. Resume with only water for a week or so. No sweat on making mistakes, gardening is one big experiment. Agree moving back to what you know and haven’t had problems with is a good plan of action. I'd still recommend going with only water for the first week or so, and then slowly introducing nutrients. If no sign of improvement observed, go back to water. I still think your deficiency is from an excess of something else, and not a genuine deficiency. Pro-Mix also over time can drop in PH, so you might need to top dress to correct it.
Plants that size wouldn't hurt being in 10+ gallons, especially since they're outside.
Agree, but that doesn't equate a lack of presence in the nutrients being fed or the soil. An NPK of 15 - 30 - 15 is plenty. The cause is more likely a lockout from an excess of others. Which is why I'd advise correcting the water problem, giving the plant time to unlock and uptake nutrients appropriately, and then reintroduce them gradually if nothing improves. The
Cal-Mag certainly isn't helping since an excess of Calcium is one thing that can lockout Potassium.