I cant answer the 12/12 question because I dont know. I wont tell you anything im not sure of from my own expeierence.. Im sure you can appreciate that.

As for the temp.. 90 is better.. but this is (contrary to what some believe) not an exact science. I would go ahead and try nutes again if you can keep the temps down but watch closely because you are still right at the limit (95F according to most).

Ideally - 78F but if you are like me that is a pipe dream.. my A/C for my house isnt even that cool.. if it was good god my electric bill.. anywho.. my room stays between 80-85 in the day and about 75-79 at night.. I am in a super hot climate so right now thats the best I can do.. but I keep my eye out for "Window unit for sale" adds and I will be getting one before I switch from flouros to the 1000 watt HPS that came in 2 days ago

To answer the humidity question.. I will caution you that I dont have the years of expierence that others here might and my grow area is enclosed inside a closet with the humidifier inside it.. but most likely I would guess that you are in a hot climate running the A/C constantly more or less which dehumidifies the house. I would def cover the walls at least or put a fresh coat of paint on them.. I would venture to say that one crop cycle wouldnt do damage and at least if you were to just use it for a bit.. well hell.. first go get a $5 hygrometer and figure out where your humidity is at.. this may be a null and void point if your humidity is already high.. but if you run the A/C most of the time it will probably be low.

Im lucky because I rent.. so I dont care about mold