1.) Is breaking at the poll essential during beginning stages of training?(she knows basics)
It is incorrect to be focusing on the head. When all is right in the world 'behind', the head looks after itself. The head falling freely from the poll is the result of engagement. So, in the beginning, you will not have it because the horse will not be strong or supple enough to achieve it.
2.)Will it come naturally through getting more impulsion, or do you have to have your horse on the vertical to get the impulsion?
Impulsion is something different than I believe you think it is. Engagement first, THEN impulsion. Impulsion is the change of 'forward' energy into 'upward' energy.
Again, the focus is not on the head, but on the hindquarter.
3.)how should I go about teaching it, because of some previous posts about being on the bit made me weary that the trainer at my barn is teaching me to do it wrong.
You don't teach it...as per what I've said above. It is a result of correctly riding the horse haunches.
4.)Should I be teaching her to give to the bit vertically?
First forward, then the training scale: rhythm/relaxation, suppleness, CONTACT (that's 'acceptance of contact, NOT 'on the bit'), impulsion, straightness, collection.
The head WILL be where it needs to be if you follow the training scale.
She say's I should hold until she gives and then release, to get her to give to the bit, and this has sounded right to me until I started reading up on dressage, and it confused me because it sounds like the horses frame should come naturally through impulsion, but I may have misunderstood, as it may have been talking about through the body (i.e. rounding of the back..etc.) ONLY.
The horse should 'seek' contact, not have contact 'taken'. When the horse is forward, relaxed, rhythmic and supple...the next step - CONTACT (acceptance), will be there. It is at THAT time that you may 'test' that acceptance by doing the stretchy trot circle; lengthening the horse's frame, then bring the horse's frame back up - the horse should not change its balance, cadence, tempo. If the horse does not follow the lengthening rein down, or the shortening rein back up, then the horse does not seek contact and therefore does not accept contact, therefore is lacking in one or a combination of: forward, relaxed, rhythmic, supple.