For your first question, I never recommend trotting then asking for the canter in front of the fence, this teaches rushing. The advantage to poles in front of the fence is the horse is taught to wait to the base, make sure they are correctly spaced first. I don't think it has to do with the timing, it sounds like she has your number right now. She knows if she gets the reins from you she can do whatever she wants. Work on your flatwork and pole work, get her on your aids, making her half halt, doing lots of transitions, and collect to lengthen work. Try trotting a fence, halting, then walking off. Make her concentrate on you.