I believe a teaching a "lean" forward in front stance is not a proper understanding of the principles of Matsubayashi. One of the keys to striking in Matsubayashi is sinking or sitting down on techniques. In front stance when you "sink" your pelvis gurdle is tilted forward slightly. For proper alignment of the spine relative the the hip gurdle there appears to be a "lean", but due to the physics of the sinking this alignment requires a slight "lean." Simply leaning without sinking properly is improper technique. As far as koshi and gamaku in karate you move with your hara. Movement with hara requires development of gamaku through movement of the koshi. Once the gamaku is developed sufficiently movement is accomplished using gamaku and any movement of the koshi is a byproduct of using gamaku.