The BEST way to coordinate management of multiple related projects is to establish a program. A program is a group of related projects that are managed together to achieve specific strategic business objectives.
References: CompTIA Project+ Study Guide Section 2.2.1
The organization should establish a program to coordinate management of multiple related projects. A program is a group of related projects, subprograms, and program activities that are managed in a coordinated way to obtain benefits and control not available from managing them individually. A program can help to align multiple projects with strategic goals, optimize resources and risks, and deliver value to stakeholders.

A ticketing system is a tool that allows the project manager to assign, track, and manage tasks for the testing phase. A ticketing system can help the resources to remain at their desks and only react when needed, because they can receive notifications, updates, and feedback on their tickets through email or other channels. A ticketing system can also help the project manager to monitor the progress, quality, and issues of the testing phase, and to generate reports and metrics. A ticketing system is different from a videoconference, which is a tool that allows the project team to communicate and collaborate in real time through audio and video. A videoconference is not suitable for allowing the resources to remain at their desks and only react when needed, because it requires their active participation and attention. A ticketing system is also different from a simple messaging system, which is a tool that allows the project team to exchange text messages and files. A simple messaging system is not effective for assigning, tracking, and managing tasks for the testing phase, because it lacks the features and functions of a ticketing system, such as prioritization, categorization, status, and history. A ticketing system is also different from a corporate social media, which is a tool that allows the project team to share information and ideas through online platforms. A corporate social media is not appropriate for allowing the resources to remain at their desks and only react when needed, because it may not be secure, reliable, or professional for the testing phase. Reference = CompTIA Project+ Study Guide: Exam PK0-005, 3rd Edition, Chapter 10: Project Execution1; CompTIA Project+ Certification Study Guide, 3rd Edition, Chapter 10: Project Execution2; The 15 best collaboration tools for productive teams3

See the solution below in explanation.

When a critical ERP system upgrade encounters a validation issue and is at risk of exceeding the allotted downtime, the project manager should begin executing the rollback plan. This action is taken to ensure that the system can be restored to its previous state without exceeding the downtime limit, thus minimizing the impact on the business operations.

The project manager has taken a risk mitigation approach by buying the item from another vendor who can
supply it immediately. Risk mitigation is a strategy that involves reducing the probability and/or impact of a
negative risk to an acceptable level. By purchasing the item from another vendor, the project manager has
reduced the impact of the delay on the project schedule, even though it may have increased the project cost.
Risk mitigation is different from risk transfer, which involves shifting the responsibility or burden of a risk to a
third party, usually through a contract or insurance. Risk acceptance is a strategy that involves acknowledging
the risk and being prepared to deal with its consequences. Risk sharing is a strategy that involves allocating
some or all of the ownership of a risk to another party who is willing to take on that risk, usually for some
form of incentive or reward. References = CompTIA Project+ Study Guide: Exam PK0-005, 3rd Edition,
Chapter 7: Risk Management1; CompTIA Project+ Certification Study Guide, 3rd Edition, Chapter 7: Risk
Management2