I have asked question number one before and its ok as long as its sealed. With the 5 gallon jugs and evaporation Im sure it wont sit around too long. Well not long enough to worry anyway.

I might get shot, but I mix my Oceanic salt first before water changes and then go to the tank and clean as well as siphon getting it all ready and then I use the water. I have been doing it manually this way for years and I have seen no problems. I also keep up on my water changes to @15% every 2 weeks in a 100gal.

Temp and Salinity are the same as it is coming from a rubbermaid container .