P.M.
A., I'm concerned about the way you describe your daughter as "stubborn," which immediately suggests a power struggle. Your description of yourself as "giving in" and practicing bribery and other "tricks," plus several other phrases, deepens my concern. And further your "lack of trust" for other "empty promises" she has made. All of these descriptors point to an adversarial relationship, when ideally, the two of you should be on the same team. (Of course, that's the ideal, which is seldom fully realized.)
I can surely understand your strong wish to have her trained by the time the baby comes. My counsel is to soften the demands, state your needs, and make a request. Explain that you'll be tired and very busy, and will need your daughter's help and cooperation when the baby comes, and that the more she is able to cheerfully cooperate, the more time you'll have to spend doing things she likes. She won't believe or remember this easily, so it's a talk you'll need to have over and over in the coming months.
It's also helpful to try to understand what needs of her own she is trying to meet with her "no-poop" strategy. More autonomy and independence? A resistance to being rushed? A means to get more of your attention, even in a strained/unpleasant way? A passive-aggressive way to express anger or frustration? I would be fascinated to ask your daughter why she stopped pooping in the potty in the first place, if only she could explain. I'll bet there's an important clue there.
A change in your approach may or may not result in your hoped-for deadline, but it sounds like you're not likely to meet that goal through your current approach. Try giving your bright daughter more choice, and explain what the natural consequences might be, in terms of her own comfort and cleanliness, and in terms of having a peaceful and happy time when the new baby arrives.
But too much urgency and stress on your part as you work this through will probably only result in more resistance from your daughter, because it will probably only increase her need for whatever she perceives she's not getting now. Add to that the typical regression that older siblings experience when a new baby arrives, and I worry that you're expecting a more mature response than your daughter will be capable of achieving.
My best to you both.