This makes the most sense to me. If I were in the OP's position, I would start discussion with the direct manager as early as possible - and begin by breaking

This makes the most sense to me. If I were in the OP's position, I would start discussion with the direct manager as early as possible - and begin by breaking down the remaining scope in terms of where the features are in this diagram. If the product must be "ready to ship", clearly some corners will need to be cut - especially the bottom right.