I've been reading up on spirits and stuff. Here are a few minor notes:
1) Bound spirits are blind to the outside world except that which they are touching. They cannot target spells unless they have Second Sight cast on them or are in Mindlink with someone (e.g. an allied spirit like Kytillae). But they can hear verbal commands from their owner if he is in contact with them. Hence Caernak can shout "Dart like a minnow!" and if Minnowsnatcher is happy, he'll cast Mobility 5. If he's not happy, Caernak needs to have a little chat. Mindspeech with a bound spirit can tell it exactly what to do as long as you can speak its language (i.e. you can train it simple tasks, as Caernak has done). Bound spirits can only Heal people or Bladesharp things that their matrix is directly touching. They can only vary the kind of spell cast if you train them to do so; e.g. different code words for Heal 1, 2, 3, etc.
2) Technically, Idylla is not a priestess of Firshala. Firshala is just a minor spirit cult and thus does not have priestesses. You might still call her a priestess, but she is really just a chosen leader. Certain of Firshala's powers will remain inaccessible to all worshippers until her cults grows and uncovers forgotten myths.