With the growing capabilities of Large Language Models (LLMs), their application to software engineering tasks has garnered increasing attention. While our previous work focused on automated review of software design documents composed of text and tables, diagram-based artifacts such as UML or screen transition diagrams remained outside the scope of analysis. Reviewing such documents requires accurate interpretation of structural and semantic elements, including nodes, edges, and transition conditions. This paper proposes a hybrid diagram-understanding method that leverages multimodal LLMs together with diagram structure extracted from Office Open XML (OOXML) representations, guided by Chain-of-Thought prompting, to incrementally interpret diagrams in real-world design documents. The method enables integrated analysis across diagrams, tables, and textual content within the review process. Evaluation results show that the proposed approach achieves high accuracy in tasks involving structural recognition, such as consistency checking. However, the results also highlight persistent challenges in semantic-level review tasks, particularly in interpreting complex transition conditions. These findings provide insights into the current capabilities and limitations of multimodal LLMs for high-stakes design document reviews.