Stage Plans in PRINCE2are essential tools for the Project Manager, assisting in the day-to-day management and control of the project. Each Stage Plan provides the detail needed for each management stage of the project, including what work will be done, when it will be delivered, and who will be involved, thus facilitating effective project governance on a stage-by-stage basis. They do not necessarily match the duration of the Project Plan or get produced simultaneously with the Project Initiation Documentation.
References: PRINCE27 Foundation handbook and other official documents.

According to the 'manage by exception' principle in PRINCE2, when any forecast indicates that tolerances are likely to be exceeded, this issue must be escalated to the project board for decision-making. This principle supports efficient use of management time by only involving them when necessary, such as when predefined thresholds are anticipated to be breached.References: PRINCE2 7 Foundation handbook and documentation on the 'manage by exception' principle.

In PRINCE2, the Executive role is responsible for ensuring that the project is justifiable in terms of cost.
This includes making decisions about whether the project remains viable and desirable within its cost constraints. The Executive is tasked with the overall direction and management of the project from a business perspective, ensuring that it delivers value for money.
PRINCE2 7 Foundation resources and materials.