... anyone explain why there are three Youmus and no Myons?

The three Youmus are Youmu at different ages. The lack of Myons is probably just an artistic choice.

Long-haired Youmu looks good...

And Satori is here... seriously, is she just roaming like her sister or will she be important later on?

Satori looks a bit, awkward the way she's standing, or "walking"

Long haired youmu with ponytail looks more like a samurai than the short haired one.

hmmm i like short-haired youmu better, but i don't hate the long haired youmu.