It's been very rewarding to see you have success with your system especially considering the work and thought you put in to it. I look forward to seeing more pics.
I would not do A because they won't be just in the dark, they would be in the dark with their feet wet. They will soak up water to the max but not transpire it as much as under light. For me, it would be a quick way to get budrot (though I got to add that shit is just in the air here). So I don't turn off the light until they are chopped, and then they go into the dark and don't abruptly stop just because they are chopped. I do think it's best to harvest after a night but multiple days I would avoid in hydroponic anyway.
As for B, during both the high ppm and low ppm runs I always ended very low, letting the res (nutes) deplete or help that by adding water.
There's flushing and there's flushing... Which is still an understatement. Flushing, one of the acts that could be described as flushing anyway, in nft works rather well. If you would empty the rez, fill it with RO water, and recirculate through that root mass it definitely won't come out at 0ppm. So you can effectively flush nutrients from the root space, much more effective than trying to leach nutrient elements from soil or any other medium. That will force it to use its reserves, of which they got plenty to last to sunday.
Long story short, I would no longer add nutes and would let the ppm drop towards 300 or less for the past 2-3 days. Not because otherwise the bud will taste harsh or w/e, but because I'm cheap and it saves nutes and it won't cost a gram.
I usually don't put a lot of value in what nutrient manufacturers say but H&G does recommend a flushing period of 2-3 days if I remember correctly for the hydro and aquaflakes. Whether it helps anything is up for debate, but giving them only water for the past several days isn't going to affect yield or quality negatively either.