I have no experience with this particluar injury, but my cats have had occasional accidents. They can be surprisingly resilient, and things can be healing inside, even if you aren't yet seeing it on the outside.
If the worst happens and you do have to amputate, take a look around the web. Many animals who lose a limb develop ammazing coping strategies, and can get around almost as easily as if they were never hurt, once they adjust to the new feeling.
But don't give up hope yet. My cat hurt the base of his tail once, possible nerve damage and the vet said if he didn't recover feeling and movement the tail would have to be amputated to prevent constant injury from being dragged. What looked like permanent nerve damage turned out to be severe bruising. It took something like two months, if I remember right, for the bruising and swelling to go down and Seuss's tail to start moving normally.
Your kitten had some pretty serious injuries, and it has only been a week. Things like this take time and patience to heal. I know it's hard to wait and see when you pet is in pain