yes, to the second. The Sponsor "owns" the scope, schedule, and budget, even if the project manager owns making it happen.
First, you should lay out the criteria for when to invoke formal change management (i.e. the triggers). It usually has something to do with changes that could impact the project's scope, schedule, budget or quality.
Second, you should create the Change Request form, that provides all salient information is documented, including alternatives considered, trade-offs and risks.
Third, you should create the process (i.e. best practice), that allows the project manager to navigate the challenge in a predictable and controlled manner. It should provide for the right mix of stakeholders to be part of the analysis of each Change Request, and the resolution of differing opinions as to the impacts of the Request.
Lastly, it should be approved by key stakeholders, enroute to the final approval by the Decision Maker, typically the Sponsor.