Please don't think I am trying to insult your or anybody else intelligence but I am going to try to explain it in it's simplest form.
the front leg of a dog has two bones, the ulna and the radius. These bones are tied together at the elbow and at the ankle by ligaments. Each bone in it's self is week, but together they have strength.
At the ankle there is the foot with 4 toes. Now Scarlet has both the ulna and radius, however, they are tied at the elbow with ligament but not at the ankle. She also has all 4 toes, only her foot is split into two with one toe attached to the ulna with ligament and the other 3 toes attached to the radius, but again the ligaments there are week and these three toes somewhat flop (no strength). When Scarlet puts her weight on that leg the weight tries to seperate the bottom of the ulna and radius even more, kind of like a wish bone all of her weight is on one side of the wish bone. Since Scarlet does not know she is handicapped and runs full bore with the two healthy puppies, her sibblings, my concern isshe is running on one toe and one of the two bones of the foreleg. I believe this is putting strain on the ankle joint and the ulna bone it's self.
Now the article I read from Canada, they operated on a Siberian Husky with a the exact same problem. They put a stainless steel plate in the foreleg and screwed the two bones to the plate. This transfers the weight form one bone to both bones increasing the strength of that leg. They also reconstructed the foot into one solid foot instead of two partial portions of a foot which takes the weight off of one toe.
If there is such an operation that could make things better for Mizzy then I am willing to spent what ever it takes to do this for her.
I again must say I am not trying to insult anyone by explaining things so simple and the wish bone explains it quite well, joined together at top and seperated at the bottom.
Yeah, I think you filled in some big gaps, with this explanation, that were puzzling folks trying to follow along.
From the picture you posted, and prior to your explanation of Mizzy's internal anatomy, it just looks like her body has an extra "bonus" appendage there, and since she is running "full bore" on it, as you say, surgery sounded a bit drastic.
As a human who has endured a couple big ortho surgeries, I can't say I recommend it as a lot of fun
If, however, she could have all the benefits you're describing, then it may be worth it. One other thing I'd investigate before making your decision, is to find out what quality resources--if any--you have in the area for canine PT.
We have nearly nothing where I am, and it's a problem. Even if you take her somewhere for a great surgeon, the aftercare, I'd imagine, is going to be about as important as the skill of the actual surgeon.