That is such a broad question that it really cannot be answered in this Reddit. The questions asked could easily be the basis for a PHD Thesis, and probably have been. Each of the questions you ask could easily overwhelm this Reddit. I will refer you to two books I have found useful in understanding the issue of chemical warfare in WWI-- They address each of the questions you ask. "Gas Attack/Chemical Warfare 1915 to the present", by William Moore, Hippocrene Books New York 1987, and "A Higher Form of Killing", by Robert Harris and Jeremy Paxman, Hill and Wang New York 1982. If you read these carefully, it may enable you to cone your questions down to those you really want to understand.