So here is the dealio on Bree's power. Because it's hard to fully explain it in-text without it feeling like information dumping. I think I've got it mostly out there, but this is just a nice concise rundown in case you were a bit foggy on the concept.
Bree is a healer. She can heal because – as a human – her desire was to just fix everything, to protect and lay low. She wanted everything to be okay and nice and neat and patched up. She wanted to be low on the radar. So that translates to her being able to fix everything as an immortal. The goal is to remain unseen and innocuous.
Within the bounds of her own body she can heal the physical as well as the mental. It's a subconscious thing and so she almost heals herself automatically. This is why her transition to animal blood is slightly smoother than others. Part of the thirst is mental, part is physical (this is my interpretation, here) so she's able to help herself out a lot without even meaning to.
After prolonged encounters with humans, Bree's thirst begins to fight an encroaching migraine. Her healing juices are kicked into overdrive as they try and quell the fire in her throat (and mind). But the thirst is powerful so you have this inner battle of sorts playing inside Bree's chemical makeup. This is why – after meeting Bella – Bree has a headache. Her power is battling her thirst and that's the result.
It takes a powerful surge of emotion for Bree to push that healing outside her own body. When Edward sort of flipped out after she told the Cullens she'd been a junkie it just pushed her over the edge. She was already nervous, that coupled with her newborn raging emotions and Edward's jibe was just the final straw. She was able to reassemble that paperweight.
She likes to use positive emotion to project her power because how much sense does it make to heal people with overwhelming emotions of anger, sadness or regret? Plus, happy emotions give more weight to her power. Positive emotions fuel the positive acting power.
Outside of herself, Bree cannot heal the mental – only the physical. Healing mental wounds is beyond her capacity. It just requires too much; she'd just end up becoming a vegetable, pretty much. Her power is also exponentially less potent. She can only force a certain percentage of it onto the subject of her healing.
She can fix small, simple things – like pencils – with ease. Because the chemistry involved in a small, lifeless object is relatively simple. Living things are more complex and require greater reserves of her energy. So she can fix a boulder in an instant but a flower or a finger is a lot harder because it is more elementally complex.
This is how she tries to help Jasper. She hopes the small amount of healing juices she can give him will quench some of the physical reactions in his system. Her reasoning is that if isn't actually as thirsty he'll calm down and some of the anxiety will go away. Almost like a domino effect. She wants it to work as a confidence booster – she thinks that's half his problem. If he thinks he's all right then he won't be so anxious and he'll have more of the 'I can do this' mentality. She thinks it's better than having Alice hover over his future and Edward his mind, just watching and waiting for him to snap. It's about striving for the good, not anticipating the bad.
When under extreme mental or physical duress, Bree's power concentrates within her and she won't be able to project it out. Her body will be trying to maintain itself and keep her from dying or having a complete meltdown so she won't be able to heal anything else. Even the elementally simple.
The wolfpack/Bree smell thing. That's tied in here too. Because her body's always trying to protect itself she has this natural force field. Sort of like other vampires have physical/mental shields (i.e.: Bella) Bree has a chemical shield. Her scent would also be hard to pick up or distinguish by other vampires. Trackers and those like Demetri would simply have to work a bit harder to find her, but a 'normal' vampire could cross her scent and never realize it.
Because the wolves and vampires are natural enemies her body cloaks itself and they could – conceivably – never smell her. She just smells sweet to them and out in the woods of Washington it wouldn't be anything that would stand out from other smells in the woods.
Conversely, Bree wouldn't really be able to discern the pack either. Her senses were extremely heightened when she first met the Cullens, so Jacob smelled a bit repulsive. That and she'd never met a wolf before. But – like the wolves – if she was on a jaunt through the woods she wouldn't be able to pick them out.
After time being in close contact they could learn each others smell. So if the pack and Bree spent a lot of time together it would become easier and easier for them to pick each other's smell out.
I think that just about covers it. If you have any questions in regards to Bree's power feel free to let me know in a PM or Review. I'll get back to you ASAP.