Just an FYIzzle, talked to a microbio professor once about a process called "bacterial de-nitrification"
This happens when you overwater, caused by resident azotobacter (who normally "fix" nitrogen from the atmosphere by metabolizing it, making it available to the plant via their biomass) who become unable to interact with atmospheric N, they then start eating any N in the soil and supposedly the plant itself, or at least outcompeting the plant for available N. End effect---yellowing leaves, also enhanced by the oxygen deficiency catalyzed by the overwatering. Just wanted to say, if you want less N just feed less from the get-go but this method of "flushing" where gallons upon gallons are dumped into the soil is just a stress regimen that may actually promote hermies and decreased flavor/potency/yield if it goes on too long.
Like wolverine said, if you have proper fade (or leaf senescence) flushing isn't indicated as necessary. I suspect the fading may be genetic, so that might expand the cases where flushing isn't needed, but I just wanted to open the concept to the overwatering angle, as that seems like more of an all-out attack on the plant to me.
My take:
If you want less N around the time of harvest, be more conservative with it earlier on---less left in the vacuole. Yes I do almost 100% water for the last 1-2 weeks as best I can estimate upcoming harvest, but no more water than I would normally use--based on what the plant is drinking. When it comes to salts etc----something really not in play if you do veganics obviously, wouldn't fret epsom myself as you shouldn't really be using much---and the heavy metals that come with them, I regret to inform you that flushing will not remove them if they are in the plant tissue and you are going to be eating them or inhaling them if you smoke the product, depending on the vaporization temperature of the contaminant.
Hope this helps,
MPP
regarding the fungus (histo)--
it wouldn't be coming through the plant tissue ie via vasculatory, rather it would be grower contact like you outlined from lack of respirator or ingestion + incubation, or consumer contact if the fungus spores get into the bud/sugar leaf.