I think most of the points have been covered but I'll chime in with what I can.
Yeah, that's not a RDWC, it's a flood and drain setup. 15 mins 3 times a day is really not a lot... your roots are likely drying out. How do they smell? because that colour gives me nightmares. You can happily up feeding to every 2 hours. Get a bigger feed pump. You want to be filling the pots as fast as possible, hold for the 15 mins, and then drain as fast as possible. It looks like you are maybe not filling very quickly, which means the level is not getting up that high. If you use a higher flow pump you should see a lot more root growth.
Reservoir: Get a chiller. Having the ability to control the root zone temp has many benefits, from the obvious keeping pyth at bay, to being able to induce characteristics during flowering such as colouration or increased resin production. Set it to about 66F Secondly, a waterfall is the best way to aerate it. Airstones are so bad in comparison when a waterfall gives you circulation and surprisingly better aeration too. If you install a chiller, it works as both, you just have the return line above the waterline. I would also suggest increasing the size of the res. Larger volume means less severe pH swings. You don't need to have the exact same pumps that came with the system. Use Newa Jet pumps, you can hook them up easily enough. I assume you have an anti-siphon on your feedpump but if you also put one on the return line, and put a sort of lazy S in the pipe, you'll not get any backwash keeping your pump spinning the whole time. I can't imagine having to babysit every feed!
Additives. You're using an awful lot of stuff there.. it's hard to place the finger on your pH problems though I suspect it's all to do with a warm res and dry roots causing rot. Drop all the gack and go back to purely your feed, and some hygrozyme /cannazym /pondclear. And only add your great white/bennies on a res change. I've heard an overgrowth can cause havoc with pH, though I've never seen it myself. It's a lot easier to diagnose problems with less variables. I know it's expensive, but hygrozym is the absolute tits... I used to use peroxide and it IS good.. but you can't use it with bennies and you do actually end up spending a lot on it because it needs adding every other day. With the enzymes you only need to add on a res change. I would also switch to Lucas'. Whilst it's not perfect for every strain it works like a dream for most. You will find you don't need to bother with pH adjustments, sometimes for the whole grow. I see people saying use peroxide - H2o2 and honestly after your nutrients, the next best thing you can do for your plants is either h2o2 or hygrozyme. Sterile or non sterile. They're both different but essentially do the same thing. I used peroxide religiously for years but made the switch to enzymes and I'm more than happy with the results. They way I see it is that peroxide cleans your dead roots, and enzymes make those roots into food. You can use them both together, but the enzymes work great with your bennies and the peroxide will just kill them.
Oh, one last thing.. if you have pyth,. you have pyth.. Next time you crop, get a big tub of peroxide and clean that system good.. pumps.. buckets.. medium (if you reuse it). Clean everything good, scrub and rinse, then have the system run for a day, with a strong peroxide solution in it. That should nuke everything.
Good luck!