Re: Kamae vs Shizentai (hips underneath)
Hip placement (kamae) is a minor part in Judo. Judo will willingly go into a space with a weakened position if it can rotate/expand/contract to create kuzushi/tsukuri/kake.
Indeed the principle of JU practically requires it much of the time. The ideal of ippon.
In Judo 'shizentai' is a tactic of defence or attack (given the presumption of conflict as a training method...even aiki uses that). A tool that can be used or disgarded according to the situation.
There is no 'stance' that is spherically sound. Once movement is part of the influence everything changes. So in 'grappling' it would be suicide to train in 'one particular hip position' (or whatever) because people would just change their training to overcome it.
Posture is composure, being settled and calm while having an in-depth ability to understand body mechanics.