One of my major issues with context validation is the behavior of a failed validation. There is no convention around what a failed validation should do to your context (besides mark it as failed). Should it add an error? To what key?

Interactor is intentionally flexible and adding that level of configurability would require Interactor to make assumptions that I don't think we're ready to make yet. We don't have enough history of typical usage and best practices to know how to handle validation failure.