The project's scope management plan will define how scope changes will be addressed and controlled throughout the project. The requirements management plan will outline how project requirements will be identified, documented, and managed, including how changes to requirements will be addressed.

Yes. The Scope Baseline - which is part of the overall Project Management Plan is used as an input to the Verify Scope process

The Project Management Plan tells you how to manage all of the different knowledge areas, and it has baselines for the scope, schedule and budget.

The Scope Management Plan is one of the subsidiary plans inside the project management plan. It has really specific procedures for managing scope. For example, it tells us which stakeholders we need to talk to when gathering requirements. It lists what tools and techniques we are planning to use when we use the Scope Definition to define the scope. And when there's an inevitable change because even the best project manager can't prevent every change, it gives him procedures for doing Scope Management. So even though the Scope Management Plan is created in the Develop Project Management Plan process, it's used throughout all of the Scope Management processes.

Drift, in project management terms, is scope creep. Scope Creep is mainly when some uncontrolled/unforeseeable changes affect the project schedule.