Who is better placed to manage external stakeholders, Project Manager, Project Sponsor or Corporate Management

Indeed, better team effort  improves handling of "Project Triple Constraint" and in turn, increases chances of project success.

When it comes to keeping most stakeholders happy, commonly changes are warranted in scope and/ or schedule (hence added costs). PM is generally constrained by the Project Charter to execute the project within defined boundaries.

If such a conflict arises, who should bell the cat.

I think it is a team effort to manage external stakeholders and that team is incomplete without the project manager who is aware of ground realities along with discrepancies etc. He can prevent being caught with your pants down and will have a technical and reasonable response to a situation.

However corporate management is equally important as they are better aware of the temperament, short and long term goals and requirements of the stakeholders. Hence a good synergy between them should present a formidable team.

Project Manager is responsible executive  the pre-determined project effectively and efficiently, he/ she has to have better relation with operational staff and internal stakeholders such as material suppliers, labor

But when comes to manage the external stakeholders such as educators, researchers, community, rivals etc Corporate management and  project sponsor would be the ideal people. They are investors and shareholders of company.