It sounds like she's been confronted for something like this, before, judging from her defensive response to you. Perhaps you should ask her advice about some of the fights, for a while, and get more of a feel for her "style", instead of the "you're great, but...approach"? Just a thought...I know I really hate the back-handed compliment, then put you on the spot, type of leadership. But then, I'm a rebel...hah!
My question to you is, why not gather the healers together, and discuss strat with all of them--just get their ideas on how best to work together. I'm not much of a raider myself, but it seems to me that your priest might be more receptive to the advice of another healer, before someone who's holding a "guild officer" stick.
There is the point you made, though, about the raid assignment. If she's to heal the raid, and you, as officer/rl, think she should not heal the tank, then simply tell her that. If she continues going oom to throwing heals at the tank, she's not following orders, and should be benched for that, or whatever action the officers deem appropriate. That has nothing to do with style--that is not following orders, and that means someone in the group who can't be depended upon.
I don't envy your role, but it is you, and your fellow officers who must decide whether to expend the time and patience on this priest, or cut her loose. It sounds to me like she's still in the "adjustment" phase of being in a new group, but, that's merely an impression. If she's trying to impress everyone with her healing prowess, that's one thing. However, if she's just a self-centered diva, then give her a "blue m&m", and gkick her out the door...
-"Just like a buzzin' fly, I come into your life, I'll float away, like honey in the sun..."--Tim Buckley