Dave,
Many taxidermists simply cut the claws off and use artificial claws in their rugs or mounts. Personally, I don't like the look of artificial anything and prefer to use the natural parts even though many times they are rough and cracked.
I have not used string but I can see where you could make it work. Tie a non-slip knot around the "fingers" and use that for circumferencial leverage. Pull on it as you evert the skin away from the phalanges. It is hard to do those details because it is usually done outside in cold weather when your own fingers are cramped and cold. Many times, what is needed is a third, or fourth, hand. Surgeons use assistants to hold traction but the rest of us can make handy little tools. Take three or four halibut hooks and file or grind off the barbs and blunt the end a bit. Use split rings to attach each hook to a piece of 12"-18" stainless steel chain. Then, attach those chains to a single split ring and you have a third hand that can grab hide or carcass. Tie it to something fixed and you can then use gravity or pull against it.
But, as I said, get a good pair of rounded nose surgical quality scissors for those fine little details in any animal.
Don't those skinned bear forepaws look very human-like?
I have a Shrade knife just like yours and have used it for the past three seasons. The gut hook is handy but, in my mind at least, it alters the "balance" of the blade and there is one section of the edge that does not acheive the desired degree of sharpness with the steel in my hands. For working tools I prefer the relatively inexpensive, but good quality, knives that are easily sharpened and cheaply replaceable. For field work, that is something like the Shrade and for kitchen and butchering it is something from Chicago Cutlery or Jantz. I usually leave the expensive and fancy knives at home.
Dan