Quantitative risk analysis helps to numerically analyze the probability and impact of risks on project objectives. By performing quantitative risk analysis, the risk manager can present the risks with the most impact on achieving the project objectives to the project sponsor. (Reference: PMBOK Guide, 6th Edition, p.
423)
According to the PMI Risk Management Professional (PMI-RMP) Reference Materials, sensitivity analysis is a type of probabilistic analysis that determines how sensitive the results of the analysis are to uncertainties in input variables. Sensitivity analysis determines which uncertainty has the greatest potential for an impact on the project objectives, such as cost, schedule, scope, or quality1. In this case, the risk manager should use sensitivity analysis to facilitate the sponsor's ask, as it will help to identify and present the risks that have the most impact on achieving the project objectives. Sensitivity analysis can also show how the project objectives will vary with the changes in the input variables, such as the probability and impact of risks2. Sensitivity analysis can be performed using various tools and techniques, such as tornado diagrams, spider charts, or influence diagrams3.
References: 1: PMI, A Guide to the Project Management Body of Knowledge (PMBOK Guide), Sixth Edition, 2017, p. 403 2: PMI, Practice Standard for Project Risk Management, 2009, p. 95 3: Quantitative Risk Analysis in Project Management - PM-Training

The Project Evaluation and Review Technique (PERT) is a statistical tool used in project management to estimate the duration of tasks by considering uncertainty and variability. PERT employs a weighted average of three time estimates:
Optimistic (O): The shortest time in which the task can be completed (3 hours).
Most Likely (M): The best estimate of the time required under normal conditions (5 hours).
Pessimistic (P): The longest time the task might take, assuming potential issues (7 hours).
PMI Risk Management Study Guide References:
The PMI-RMP Exam Preparation Study Guide outlines the PERT formula and its application in estimating task durations, highlighting its effectiveness in incorporating uncertainty into project schedules.

The first thing the risk manager should do is analyze the situation and meet with the risk owner. This will allow the risk manager to understand the challenges faced by the risk owner and work with them to find a solution. Conducting a cost-benefit analysis or changing the risk response strategy may be necessary, but it is important to first understand the situation before taking any action.
According to the PMI-RMP Exam Content Outline, one of the tasks in the domain of Risk Response Planning is to "assist the risk owners in developing and implementing risk response strategies and actions based on the agreed-upon risk response plan". Therefore, the first thing the risk manager should do is to analyze the situation and meet with the risk owner to understand the root cause of the challenges and the cost overrun, and to discuss possible solutions or alternatives. Highlighting this situation to the project manager, conducting a cost-benefit analysis, or changing the risk response strategy are possible actions that can be taken after the analysis and meeting, but not before. References: PMI-RMP Exam Content Outline, Domain 3: Risk Response Planning, Task 31

When a team member is struggling to ensure the coverage of all risks in a complex project, the risk manager should develop a risk breakdown structure (RBS). An RBS helps in systematically identifying risks by categorizing them into different levels and areas, making it easier to ensure that all potential risks are considered. This structured approach is particularly useful in complex projects where risks may arise from multiple sources.
PMI advocates the use of an RBS as a best practice for comprehensive risk identification, particularly in large or complex projects where the range of potential risks can be extensive.

With 25 identified risks, and considering the constraints and resources available, the appropriate next step is to perform a qualitative risk analysis. This process helps prioritize the risks based on their probability and impact, as well as other relevant factors like urgency, proximity, and detectability. By doing so, the team can focus on the most significant risks that require immediate attention and develop response plans accordingly, optimizing the use of available resources. PMI recommends this approach to ensure that risk management efforts are both efficient and effective in addressing the most critical risks first.