Question 1: Are teas and flushes necessary? I would say sorta yes. If you inoculate your soil with a decent collection of biological life, either through an instant tea or one you brew yourself, if you keep your soil moist enough they'll survive and keep eating and osmossising out things the plants love. Flushes serve the purpose of using up any nutrients left in your soil and preventing waste buildup in the root system. That's why they recommend the Feed Feed Tea, Feed Feed Flush. Now in reality, I water every other day, making it about 3 times a week. By the time I'm done with veg I've probably done upwards of 4 teas and 4 flushes. In reality it becomes more like 2 teas and 6 flushes.
Implied Question 2: How much to water. I take the Scott's (Nectar owner) advice and water them just enough to get them to when I want to water them again. So in my case I like to water every other day. So once established in late veg, I'm feeding about 1 - 1.5 qts every two days per plant in 3 gallon pots. By the time I'm in late flower, I'm usually upwards of 0.75 - 1.25 gallons per plant in 5 gallon pots. A fresh clone or seeding, 4oz or less. My pots aren't "fabric' per say, but they're like fabric, I use Rain Science bags which are more a nylon weave but they're the same principle as fabric pots, before them I used SmartPot fabric pots and didn't really have to water any more than I do now.
Yes you should be doing slurry tests. You want your soil sitting between a 6.0 - 6.8 ph, the closer to 6.5 the better but it will fluctuate; and your ppm between .7 - 1 in early veg, 1 in late veg and early flower and I've pushed as high as 1.4 in mid flower. You're using One Shot which will skew your ppm readings upwards. I would add at least .3 to your expected soil ppm to compensate for having One Shot already in. I can't really tell you what One Shot raises your PPm to but I would expect it to be about .1 - .3 higher. I always get my first slurry before even planting plants and take it before each tea or flush so I know where they're sitting before I tell them to clean their plates and if I need to feed them more. That way if I need to up my nutes, I can tell from the ppm if I'm hitting below .9, I know I need to start upping my feed. BTW most Nectar fam uses the Truncheon scale which is EC x 700. So an EC of .7 would be 490 and 1 would be 700.
A note on slurry tests, get yourself some RO water (my local grocery store sells it as drinking water) and use that for your slurry tests since your tap water is so high. That way you don't have to pH down your water and add a possible acid to your slurry test. Technically your supposed to use the water you water with, however if you're yanking your pH down already it's probably better to just us commercially available RO water. Don't use
De-Ionized/Distilled water, it can mess up your probe and doesn't give a good reading for the Nectar system.
As to your high pH, as was posted earlier, bring it down to about a 7.0, let it stabilize, then add your nutes and then bring it back up with Oly Up. Remember the limestone in Oly is as much food as it is pH up, so don't use another brand of pH up. pH down, whatever, phosphoric acid is phosphoric acid, but Nectar's is much weaker than GH, either one doesn't matter just use it before you add your nutes. I use GH for my aeroponic cloning chamber and have a bottle of Hades I never use.
I would strongly recommend any line of Mycorrhiza for at least one of your teas per stage of life. I have used both Mycos and Bigfoot, don't really have a preference, fungus is fungus. Other inoculates to consider,e SLF100 (every time I water), Photosynthesis + (Every time I water), Mycorrhiza
and the Azos Baccilum family of bacteria (at least once per life stage as a tea.)
You're in far less risk of underwatering than you think, especially if you have got good living Mycorrhiza in your soil. That fungus latches onto your roots and stores water for dry times and waters the plant for you. I'm not saying this is advisable, but I once let the plants go for a full week with just one regular watering before I left in deep flower and I came home and they were just starting to droop and hadn't lost any color or leaves. Again I don't advise it, but I feel like Myco is kind of an idiot buffer that gives you a day or two on your watering schedule if something comes up.