When faced with the inability to complete all items on the high-risk mitigation list within the remaining time, the most effective action is to communicate the current progress and challenges to the stakeholders. This allows for a collaborative decision-making process to re-prioritize the remaining high-risk items and potentially secure additional resources or time to address them.

References:The ISTQB Advanced Level Test Manager syllabus emphasizes the importance of continuous monitoring and control of test activities to achieve project objectives, which includes assessing and reporting relevant and timely test status to project stakeholders123. It also highlights the need for test managers to organize and lead risk identification and analysis sessions, and use the results for test planning, monitoring, and control2.