I am so sorry you are having such problems. I do know precuts are often not accurately cut, so that is probably what made your top not lie flat. Bias edges on pieces can also stretch, causing problems. Puckers on the back can be avoided by making sure the backing fabric is smoothed out really flat and taped securely around the edges before layering and pinning or thread basting. You are not alone--we have all been there, and we all hate ripping out as much as you do. Have done it many times before, will have to rip out many times in the future. All we can do is try not to repeat mistakes by learning from them.