Partially. After orthodontic treatment it takes time for the supporting bone to mature and reposition. During this time the teeth will tend to revert back to their original position. That is why a retainer is required. Some teeth will require long term or "fixed" retainer or the teeth will move. Once the teeth are stable retainers may not be needed. Check with your orthodontist.
Not original but... Perhaps not the "original' position, but they certainly will not stay in their final best position. Why go through all that time and money and then have your case relapse?
Sort of. When we move teeth some of the fibers that hold the teeth into the bone get stretched, and the bone also needs to readapt. If you do not wear retainers i can practically guarantee that you will have wasted your time and money. Wear retainers as directed by your orthodontic specialist if you want to 1) minimize change, and 2) protect your pocketbook.
Yes. While they may not revert all the way back to where they were, they are almost certain to get at least slightly crooked again without retainers.