The Japanese use a thumb-release rather than a three-fingered release, and as with any culture using the thumb (manchu, mongolian, korean, ancient egyptian etc.) the arrow must be on the opposite side of the bow to us - its to do with how the string slips out of the fingers upon release. As to manuscript representations, it's highly variable. Some are better than others; I have been through two entire books available as online resources: the Smithfield Decretals and the Queen Mary Psalter. The Decretals, in particular, are quite precise in differentiating between Warbows and Hunting bows, both in terms of the archer's draw-length and the size of the bow. It's perfectly evident that the illustrator knew the difference between the two bows; you don't need a 140lb longbow to shoot rabbits and other small game (what most ordinary people were permitted to shoot) and even for large Red Deer stags, 140lbs would be overkill. As a lighter bow, say 60lbs, performs really badly when left at around 74" length, it makes sense that these bows would have been shorter.
Dave