I think 2hus can shoot danmaku bc they have a special "danmaku organ" located in the abdominal cavity that lets them shoot them, it also gives them levity so they can fly. Reimu had a defective danmaku organ in the PC-98 games which is why she needed to fly on genji but could still shoot them. she had surgery right before EoSD to enable her to fly on her own

also I think witches have a witch-organ that enables them to do magic, I think it would be real funny if Battler threw a ball at Erika & it hit her in her ribcage & she were to fall over & clutch her side & cry "ow! my witch-organ!" then she tries to do magic to get revenge on Battler but bc her witch-organ is ruptured the spell backfires & she explodes

this would imply that Marisa has both a danmaku-organ & a witch-organ; however, bc she needs the broom to fly, her danmaku might be magic, not natural, meaning she might not have a danmaku-organ at-all