The Quality Management Approach in PRINCE2 should provide guidance on how and when prototyping is to be used within the project. This approach outlines the quality techniques and standards to be applied and the responsibilities for achieving the required quality levels. Prototyping, as a method to validate product quality and user requirements early in the development cycle, falls under these guidelines.References: (PRINCE27 Foundation Handbook, AXELOS)

The risk response type that is applied on a pain/gain basis is 'Share'. Sharing involves partnering with others to share the impact of the risk. This is often used for both threats and opportunities where sharing the risk can enhance the capability to capitalize on the opportunity or mitigate the threat, with potential gains or savings being distributed according to an agreed formula.
PRINCE2 7 Foundation handbook and documents.

Explanation/Reference:
Reference http://prince2.wiki/Quality

The 'managing product delivery' process in PRINCE2 defines the work needed to deliver major products, ensuring effective use of organizational resources. This process focuses on the acceptance, execution, and delivery of project products according to agreed plans between the project manager and the team manager.
References: PRINCE27 Foundation guide on the 'managing product delivery' process.