1) depends i you're growing in soil or hydro - hydro i use 5.2-5.8, sorry can't remember soil ph window and would rather not blag.
2 and 3) If your water is hard (ph 7+) then you should lower the pH with pH down (nitric/phosphoric acid) and raise the pH with water. And if your water is soft 6ish, then you should raise the pH with pH up (sodium hydroxide) and lower the pH with water or if you need it lower than 6 use pH down. but NEVER USE pH UP AND pH DOWN IN THE SAME RES - insoluble salts will form. However, if you are in soil then you strongly influence the pH with the soil medium - once again i dont do soil, dont wanna blag.
4) you'll kill the plants with extremely low or high pHs (ph 1-4 or 9-14) but nute lockout is the main reason for pHing your water - to keep your nutes soluble.
5) If you are in hydro, change your resovoir - only reliable way to do it. And if you're in soil.. dont wanna guess.
6) Not sure mate sorry.


Hope that helps, hope you've got a hydro setup or it wont lol.