Onions play an important part in the thick gravy. Another key thing is keeping the heat on high for the initial 10 minutes so that the chicken roasts and the gravy gets thicker which gives it an amazing flavor.
If you think its too much work feel free to avoid the excess mixing and enjoy the curry with gravy.
Its fine to use both curry leaves and coriander leaves, but for an authentic keralite dish, curry leaves are used.
Try not to overcook the chicken to a stage where it falls of the bone. Bone in chicken is recommended. (Stores like Publix will cut the chicken or any meat for you the way you prefer).