Leaves will die as the plant takes it's energy from them, perfectly normal in the final stages, doesn't mean they are ready to harvest.

If the flowers keep getting bigger, let it grow. Once it seems that they are not getting any bigger, wait a few days and then cut it. I would harvest the plant at one time, because excessive trimming can stress the plant and slow growth.

I don't know what you mean about water condition. 2 weeks before harvest you should use only plain water. You can also stop watering a few days before harvest to make the drying process faster.