I am using 12% bleach, kept it in the dark. However I am concerned of its shorter shelf life than 6%.
If I dilute with water for storage, will shelf life be extended or I will have some sort of reaction that maybe hazardeous ?
As long as you prevent the chlorine from getting too warm, then it should still last a reasonable amount of time as shown in the chart at the bottom of this link for high quality chlorinating liquid. I use 12.5% chlorinating liquid from my pool store and get about a month's worth (4 gallons) at a time and I barely notice any difference at all at the end of the month. The average temperature in the hottest part of the summer where I live is around 70F so the half-life is around 315 days so after a month the concentration should drop to around 11.7% so not a big deal.