Can I use salted butter and can I use yogurt instead of buttermilk

The reason unsalted butter is used in baking is so you can control the amount of salt in the recipe. If all you have is salted, it’s ok to use it, but don’t add salt. Using yogurt in place of buttermilk is usually fine. It will result in a somewhat thicker batter and frosting. That said, the buttermilk is an integral part of both the frosting and the cake, so it might be worth a run to the store for buttermilk and unsalted butter if you really want to make this recipe.