Well, im not exactly a genius in growing... But i have problems like you before. From my experience i could say this: (post1) feeding the plants with too much nutes , (post8, 2nd picture) too much nutes, (post8, 3rd, 4th picture) feeding the plants with bad water or overwatering. I experienced this rotting from the middle of the leaves, when i was using rain water which was in the container for quite a long time. 2nd time when i experienced this problem, i used also old water with added nutes. Few other times happened to me, when i was overwatering. My advice would be that you feed your plants with fresh water next couple rounds. You need to put water in clean tank over night, so all the clorine evaporates. If you tap water is safe to drink, then its safe to use it for feeding plants. Thats what i would do...
In your case i would say, that you used too much nutes and then you tried to fix it with flushing, which brought you another problem.
My opinion on flushing: You need right combination of soil mix and right enviroment, so the soil doesnt stay soaking wet too long. Strain genetics is also important. I avoid flushing at all costs, cause i do more damage then good to the plant.