Accelerated urbanisation and associated land-use conversion are reshaping the composition and functions of terrestrial ecosystems globally. In Southeast Asia, ecological change is increasingly mediated not only by demographic urbanisation but also by urban expansion, peri-urban development, and the conversion of agricultural, coastal, and forest land into built-up surfaces. This study integrates multi-source geographical information from 2014 to 2024 to examine 351 provincial-level units in 11 Southeast Asian nations. To describe the spatial-material dimension of urbanisation and ecological conditions, two indices were created: the Composite Nighttime Light Index (CNLI), used as a proxy for urban expansion and built-up development intensity, and the Improved Remote Sensing Ecological Index (IRSEI), which is tailored to tropical coastal locations. The development of human-environment interactions was measured using the Coupling Coordination Degree (CCD) model. Pathways of synergy and trade-off were found using an incremental four-quadrant framework, and nonlinear causes of spatial differentiation were investigated using Spearman correlation and the Optimal Parameter-based Geographical Detector (OPGD). Uncertainty was addressed through data-quality masking, annual compositing, consistent index-construction rules, and cautious interpretation of CCD and driver results as relative provincial-scale patterns. The regional mean CCD rose from 0.250 to 0.314 during the decade, showing a slow improvement; nevertheless, most places still have low to moderate levels of coordination. There is clear pathway divergence, with 38.7% of locations enduring trade-offs where built-up development happens at the price of ecological quality and 58.4% of regions seeing synergistic improvement. The coupling pattern is primarily driven by built-up area expansion, with multiple factors jointly producing strong nonlinear enhancement effects. Climate conditions and forest disturbance further strengthen these effects. This study extends beyond single-country analyses by situating remote-sensing coupling results within land-use transition, peri-urbanisation, urban–rural linkage, and regional-governance perspectives. It provides quantitative evidence to support differentiated policy strategies in rapidly urbanising places.

The conflict between individual rationality and collective rationality, as revealed by the tragedy of the commons, constitutes the core dilemma of collective action in ecological and environmental governance. The key to resolving this dilemma lies in systematically diagnosing the social–ecological system and identifying the crucial factors that hinder cooperation. However, existing diagnostic frameworks have largely focused on identifying the conditions for collective action formation. Thus, those frameworks establish a diagnostic logic centered on conditional principles, but they neglect the capture of the formation process of collective action. To address this gap, this paper further introduces a procedural principle centered on “squeeze-out → transformation → return” into the traditional diagnostic framework. This is achieved by constructing an integrated analytical framework that combines procedural and conditional principles. This study takes the ecological and environmental governance of the Dawangtan Reservoir in Nanning, Guangxi Zhuang Autonomous Region, China, as a case study. The limitations of the traditional framework in diagnosing complex social–ecological systems are also examined. Further, this study demonstrates how the proposed integrated diagnostic framework enables problem identification and dilemma resolution by capturing the formation process of collective action. This research not only enriches the theoretical understanding of the formation process of collective action in complex contexts, but also helps practitioners more efficiently identify context-specific solutions to collective action problems.