What is the correct way to put an organization on the proper path to changing their mindset to an agile approach?

During project completion, a project manager receives a request from the customer for a reduced cost of goods. The customer's request will affect the sales department's budget and forecast.
In order to reach a consensus and have a successful project completion, what should the project manager do first?

An intern is replacing a critical resource midway through a sprint To help the intern get up to speed, the project manager assigns a senior engineer in the team as the intern's mentor Two months later, the project goes into critical status, delaying the release milestone by a month.
What should the project manager have done to avoid this scenario?

A team member has encountered a technical issue, similar to the one faced in a successfully closed project. However, the team member did not know about the previous project and would like to resolve the issue on their own.
What should the project manager do?

During regular project progress review meetings, one team member shares bad news that has a significant impact on the project schedule. How should the project manager react to ensure the project remains successful?