If you have any leftover high-temp mortar, tape it closed and place it in a MARKED trash bag (to keep out moisture). Save this for a rainy day - it will last about 3-4 years before turning rock hard.
Now, here’s why you don’t want to use high-temp mortar over the stucco… High-temp mortar can only fully cure once it’s subjected to high temperatures (like fire). If you apply high-temp mortar as your first layer of the exterior shell, it may never fully cure and you may have a product failure
DO NOT use leftover high-temp mortar for your shell.