Thank you very much for your questions, and unfortunately to the first question whether we can select another posture other than the Full Lotus Posture or Half Lotus Posture, my answers are negative.
Because the Full Lotus Posture or Half Lotus Posture have been used since when Gautama Buddha has begun the practice of Zazen, and such kinds of methods haven't been changed for about 2,500 years in Buddhist Societies at all. Therefore if we want to change the method of practice, it suggests that we want to change the Gautama Buddha'practice itself, and so it suggests that we want to change the Buddhist teachings themselves. Therefore it is completely impossible for me to avoid the method of Half Lotus Posture or Full Lotus Posture.
Even though in Soto Sect recently they sometimes recommend for secular practioners to use chairs for Zazen, but I think that such a kind of idea might be serious rebellion against Gautama Buddhas' teachings.