Japanese used mobile pavises, aka Tedate. In defense traditionally the whole front line of an army was covered with them like in this picture:
It is true that majority of wounds and casualties came from ranged weapons. Professor Karl Friday has done some quite extensive research about it by examening human remains from the period.I dont have the exact figures here right now, but his study suggest the same that ranged weapons made the majority of wounds.
About the organisation of archers or any units.As far as i know.The smallest organisational unit in Japanese army of the period was "kumi" roughly ten men.
About the fighting on horseback. We have to remember that only samurai were mounted.Ashigaru were never allowed to be mounted warriors. So those who fought mounted had trained all their lives for that.Also the Yari spear carried by the cavalry was lot shorter then infantry version. Last the Yari was not only used for stabbing if you think of a mounted warrior.As it had a long blade sharpened from edges also.It was used to slash, just as well to stab. So i would dismiss the idea that you could only wield Yari with both hands and we have several depictions of samurai fighting mounted.
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