Is she an experienced traveler, able to navigate connecting flights and immigration on her own? If so, she could fly to Santiago (via connections somewhere in Europe) and you would meet her there. But make sure the journey is booked all on one ticket so it is guaranteed by one airline, in case of missed flights. Or, she could fly from the US to Madrid and you would go there.
The last-minute airfare for this trip would be high - it might almost be the cost of another trip. It would basically involve abandoning your own plans, which I'm sure you would happily do if it were either necessary or truly beneficial for your granddaughter. However, it might turn out to be hectic and not fully satisfactory. I don't know you, your granddaughter or any of the personal dynamics that are involved.
I understand your enthusiasm, but it might make more sense to plan another dedicated trip in a few months, if you can make time for another 2 week trip. That way you would get two caminos for you, and a trip planned just right for her.
Just my immediate thoughts.