Juhani is okay, but why is she on the light side when she uses dark side powers? As I understand if your on the light side dark side powers cost more and vise versa?
Also is there a way I can make a party member more attuned to the light or dark side or is the player character the only one who can change?
you can change it, with cheating. control the character you want to add darkside/lightside too then do: adddarkside # (replace # with number 1-100) addlightside # 100 being the highest and 1 lowest. Oh make sure you've enabled cheats as well.
Juhani deals on darksided powers because of storyline development. If you recall, she "fell" to the darkside by attacking Quatra. This may be consistent with the plot yet plays foul in game mechanics, for as you have stated, her powers spend too many Force Points. I would suggest you to get the ever useful KOTOR Save Editor and make any modifications on light or dark tendencies, though it would make the experience less believable now, wouldn't it?? =p
I don't mind. I don't really use Force powers. Even when playing Dark side, I always go for Heal first. Anything else, i don't really care about.