<p>The persistent gap between pedagogical theory and classroom practice continues to remain a prominent issue within pre-service EFL teacher preparation, especially in Confucian-heritage contexts characterized by hierarchical norms and collective learning traditions. To address this, we conducted a three-cycle Design-Based Research intervention from September 2023 to June 2024 with 120 junior-year pre-service teachers at three Chinese normal universities. Participants engaged in a sequenced multimedia case library integrated with a structured 3C mentoring model, comprising case analysis, collaborative discussion, and contextualization, which are closely linked to the application of Cognitive Flexibility and Experiential Learning. Data sources included a 20-item pre- and post-intervention skills test, a 15-indicator mentor observation checklist (<i>N</i> = 600 entries), 360 reflective journal entries, 480 scenario worksheets, and platform engagement analytics. Following the intervention, decision-making accuracy rose from 65.4% to 87.2%. Convergent quantitative evidence comprised significant gains in observation scores, scenario-worksheet performance, and increased log-ins and time-on-task, with repeated-measures ANOVA confirming progressive improvement across cycles. Thematic analysis of journals and worksheets revealed 68% reflective observation, 59% peer-scaffolded schema restructuring, and 49% contextual adaptations, alongside frequent emergence of “pedagogical third spaces.” These positive observed changes are closely associated with the implementation of the integrated multimedia case library and 3C mentoring arrangement, showing close linkage with narrowed theory-practice divide. We conclude by advancing 12 evidence-based design principles for scalable, culturally attuned EFL teacher training.</p>