Yes it is a 1:48 twist. The Seneca is a fine rifle. I owned 2 of them. They are a joy to carry and the .45 is plenty for deer if you're so inclined to use it. As for the wedge pin. I normally put a slight bend in mine until I get the tension I want. When I do, I put a "T" on the top so I know which way it goes in every time.
As for the crack - I good professional grade wood glue will hold it together. Just CAREFULLY put a knife edge or thin screwdriver tip in the crack to spread it slightly and force the glue into the crack filling it completely. Once filled, remove the tool and lightly clamp it to hold it in place.
Nice rifle - enjoy.