Q-NaGreen plums and ripe plums make two different drinks
The same fruit gives two results. Hard green plums make a clean, sharp, clear drink. Yellow ripe plums make something sweeter in the nose and thicker in the mouth. Green plums hold their shape and stay clear even after a long steep. Ripe plums have thin, soft skins, so they give more aroma but cloud the liquid and need more care. Commercial umeshu follows the same split, so choose by whether you want brightness or perfume and you will not go far wrong. Soaking green plums in water overnight to draw out the bitterness is what keeps that sharpness clean.
Q-NaRock sugar is used because it dissolves slowly
Rock sugar in umeshu is not only about sweetness. Fine sugar raises the sugar level straight away, which pulls water out of the plums hard, shrivels them, and ends the process before much aroma has come out. Rock sugar comes in large lumps that sink and dissolve slowly, so the concentration rises bit by bit. The plums have time to give up both water and scent, and the spirit works its way into the fruit in exchange. It is chosen for being slow. Some people add the sugar in stages instead, which helps when you want it less sweet.