Image Difference Captioning (IDC) has produced many task-specific models, but their gains are often entangled with isolated implementations, data processing, training protocols, and evaluation settings. As a result, the field accumulates method-level scores more easily than reusable structural evidence. This paper proposes OpenIDC, a flexible and extensible codebase that reconstructs a unified evidence chain for lightweight task-specific IDC. OpenIDC standardizes data pipelines, model interfaces, training strategies, and evaluation protocols, enabling 8 representative method families with 9 executable variants across 3 benchmarks to be reproduced, compared, decomposed, and diagnosed under consistent conditions. Beyond reproduction, OpenIDC advances IDC from final-score comparison to normalized design-space diagnosis. Based on this diagnosis, we introduce SIDFormer, a compact Spatially-guided Invariant-Difference Transformer that serves as a structural answer to lightweight IDC by organizing spatial anchoring, pseudo-change suppression, and reliable difference evidence through functional fusion. Experiments show improved reproducibility and comparability, together with competitive SIDFormer performance across the evaluated benchmarks. Code, reproduced methods, and models will be released.