Alright, I'm established on another site, but I had to register for 420 so I could comment and give some advice to you Leonardo. Rather than try to address all of your questions and concerns over the multiple post pages, just hit me up with the most pressing ones....I think I can help you out.
Did you get calibration fluid for your pens? Calibration is easy as cake and needs to be done often, especially Ph. I calibrate 1 - 2 times per month.
Your PPM levels are WAY too high for plants at that stage. I'm actually shocked that you're not seeing nute burn. I'm usually in the 300 - 500 level at the stage you are currently. I max out during early - mid flower in the 1200 - 1300 range and start to see nute burn if I hit anything over 1400. Keep your eye out for burn which will start on the tips of your leaves.
Go get a bottle of neem oil for your powdery mildew problem. As I see you're in Nor Cal, I know it can be a problem with the humidity levels seen there. I mix neem oil with water in a spray bottle and spray my leaves, top and bottom, once per month and have never had a problem with PM. Neem oil is a non-toxic fungicide, miticide, and insecticide which is very convenient for protecting your plants from powdery mildew, mites, and other little nasties. It leaves a sheen on the leaves for a few days but wares off as the plants grow...no need to wash off. I have had phenomemal results and it has saved me the headache of using a sulfur burner (which can ruin bud taste if used to late in grow) or a dehumidifier, etc. Neem oil works to prevent powdery mildew by changing the Ph on the leaf surface to a Ph level which is non-conducive to fungal growth. Chances of having powdery mildew and other types of diseases like damping off, are greatly reduced when their ideal Ph growth levels are offset. Non-toxic to plants, non-toxic to humans, deters nasty bugs, prevents fungus....bingo!
Go buy a reverse osmosis filter on Ebay. You can get one that will do just fine for $100 - $150 I believe. Buying bottled water, especially for things like flushing, is no bueno.....silly really. Sorry, don't mean to criticize, but that is expensive and not very green to be having plastic bottles used up like that. R/O is an investment you will be very happy with, not only is it ideal for your plants, but your drinking water as well.
100 GPD RO Reverse Osmosis Water Filter 6 stage DI NEW - eBay (item 370244125601)
Last but not least...and I hate to say this and it may sting a bit if you have your heart set on it....ditch the Rotogrow/Volkswagen idea. Serious...
Look on the internet and the only people you see promoting these things are the manufacturers, and people who have bought them and want them to work so bad, that they work and strain and toil in order to get them to grow what amounts to be a relatively weak amount of bud.
You could easily harvest 1 - 1.5 lbs with a nice 8 - 10 plant hydro system under a 600 Watt HPS...no problem. You're already on the hydro road with your coco grow you have there. I would seriously, highly recommend a shift of thought on how you're going to proceed. A RDWC (recirculating deep water culture) system will be much easier and require less maintenance that a roto grow....trust me. I've grown with 3 different methods over the years and have finally settled on a RDWC system I built for a couple hundred bucks and it produced the biggest harvests yet. Ph is much much easier to maintain and it requires about 10 - 15 minutes of maintenance per day, but also allows me to leave for up to a week at a time with no worries. Believe me, you, being somewhat new to the hobby, will HATE rockwool. I do and many many others do as well....it sucks. Type in "Rockwool sucks" on Google and you'll see what I mean. It can be done and done well by people who have gone through much trial and error and prep work, but it is not for beginners. It gets soggy and promotes root rot and the Ph is hard to get stabilized.
I will stop rambling and see if you have any thoughts. Talk to you soon.
jcom
p.s. - let us know how it goes with the landlord....