ok, I am going to take this point by point because there are a few misconceptions that I want to clear up and you have asked so many good questions that I dont want to miss anything.
First I want to mention that all of the people you are going to ask these questions of are likely to give you different answers. None of us grow using the others style or methods. Each of our techniques have been honed by years of practice, and trying to mix and match between us is likely to be very confusing. I will give you my answers, but keep in mind that other responses may turn out to be different, and we will all be right, in respect to our way of doing things.
I feel a little strange adding additional coco to the strong designer organic soil. The designers already included some of that in there to help with oxygen retention, and adding more just seems like second guessing the folks with degrees, so I don't do that. I do add perlite, since our weeds do prefer a slightly lighter soil and over a 4 month grow a soil will tend to compact if not broken up with an abundant amount of perlite or pumice. 20-25% is pretty normal. This will not change your watering frequency and actually it will give the microlife (if you have any) places to hide.
Flushes... there is more confusion about this right now in the online world than ever before. You do not yet understand based on your description. That scientific article only said that nutes do not show up in the final buds, but it never said not to flush... it said that flushing at the end to remove the taste of nutes in the buds, was useless. I have written extensively about this, and because of built up salts and debris in the soil near the end of the grow, I insist that a final flush, right at the start of the final bud swell two weeks until the end, is necessary for 100% water/nutrient uptake into the plant during this most critical time. If you don't flush, the salts and debris in the soil will restrict your uptake to some degree... and to any degree means that your final yield will be less than what it could have been.
Now, what is a flush... did you flush? No, you really did not. A proper flush that actually cleanses the soil requires 3x the container size in water moving through that soil. You did 1x, not nearly enough.
So then someone told you to go down the rabbit hole of testing runoff PPM out of soil... and you got concerned. Don't waste your time testing runoff in soil... it really tells you very little. You were not measuring salt only... you were measuring anything water soluble that had broken down, moss that had broken down, minerals that were now freed up, carbon, salt... runoff PPM of soil is quite impressive, and means nothing.
Now, the water. Unless you are running an organic grow, or a hydro grow where you are tracking PPM, tap water complete with its chlorine is perfectly fine to feed a soil grow.. and actually the minerals in tap water can be beneficial to the plants, and even stave off the dreaded and very common magnesium deficiency that shows up so often when people use RO water. So all that using tap water first and then distilled water... totally unnecessary. You are overthinking this. Actually during a flush, since you are washing all of the nutes out of the soil, you don't even need to adjust the pH... there is nothing there but water, and the plant doesn't care what the pH is as long as it is within reasonable limits.... the
ONLY reason we adjust pH is to activate chelated nutes in a synthetic grow.
And then, after your partial flush, you fed back nutes like a coco grower would do and this confuses me. In your soil grow, are you feeding every time you water, or are you doing feed/water/feed/water like you should? If this is the case, your flush should have just taken the place of one of your plain water waterings.
So, I call a flushed soil a clean soil, not a recharged one. To recharge soil I add back in raw minerals and cook them in over a couple of months. Flushing should be done every time the salts build up, and in a FF grow with FF nutes going in at full strength, you should flush 3 or 4 times during the grow... but that very most important time is right before bud swell at the end. A flush is also not going to wash out beneficial fungi... they sort of attach themselves to the roots and you are not going to dislodge them. Beneficial bacteria will be flushed away, but there will be more to replace them, especially if you are using FF Big Bloom, which brings a new load of them every time you water with nutes.
I have grown a couple of plants that had chemdog in their bloodlines, ran that during most of 2017 with Santero's stuff. I never have grown out a pure chemdog though, that would be fun.
Lastly, all respect to
@InTheShed, but I will advise against trying to run FFOF with its strong upward drift, without carefully adjusting your pH to 6.3 every time... not 6.5, and not just going willie nillie and letting the numbers fall where they may without adjusting pH. Let me please suggest that you only try this bold experiment on a side by side test, on a plant that it wont kill you to lose... because that is what I predict will happen. Shed has some unexplained voodoo going on there in that shack, and I know that if I tried to replicate it, something untoward would happen to me... a toe would fall off or something. It is scary stuff... be sure to have backup.
