Silica is one of the most abundant minerals on the Earth and is present in almost all soil..By supplementing additional Silica, I find my stems, branches etc are stronger, tolerate heat better, and some some say, makes a bug's life MUCH harder when they attack.
Google "Silica for plants"..It is relatively inexpensive and readily available all over (Amazon, Ebay, etc).
I always stop treating with it about 5 weeks into flower.
Deciding when to flip Photos is the grower's choice..After sexual maturity..More Veg time = bigger plants= better yields. Again, IMO it is more a matter of how tall you want the plants to get as they stretch in flower. Usually stretching is over by week 3..BUT some plants come back stretching after a brief pause. Just try to plan on having the plants grow into the scrog..Then you can lollipop everything underneath letting the plants focus on buds:)
You have plenty of air from what you say (run all on low speed.)..I forgot..is there an exhaust fan as well? (Mine is a 430cuft per minute that I have turned down a bit (+ my little 6" fans in 3x3x6 ft tent)