The structural stability of barrier layers is critical for the long-term effectiveness of landfill remediation projects, although leachate pumping and organic contamination can cause structural degradation, reduce remediation performance, and increase the risk of pollutant release. The objectives of this study were to determine the consolidation–rebound mechanisms of sand–bentonite mixtures through standardized tests and to analyze deformation under diesel contamination using multi-scale approaches, including pore-structure characterization, particle-size distribution, cation exchange capacity, and oil-blocking effects. The results revealed that uncontaminated soil (0.0 wt.% diesel) exhibited non-linear compression behavior, with an initial decrease and a subsequent increase with increasing sand content; when the consolidation pressure exceeded 400 kPa, the compression rate decreased markedly. The compression deformation of the contaminated soil increased and was positively correlated with the sand and diesel contents, with accelerated deformation at >4.0 wt.% diesel. The rebound capacity decreased under combined sand–diesel effects. Microstructural analysis indicated that initial compression was controlled by inter-aggregate pores, whereas mid- to late-stage compression was influenced by intra-aggregate pore evolution and particle breakage. Increased diesel content shifted aggregate breakage from single/secondary to tertiary patterns, altering later compression behavior. Coupled hydration reduction and enhanced oil-blocking suppressed rebound significantly, worsening with increasing diesel content. Technical–economic analysis revealed that pure bentonite (0% sand) was optimal under uncontaminated conditions and that a 10% sand mixture was best under contaminated conditions. The sand–bentonite barrier exhibited amplified consolidation–rebound deformation and reduced stability with increasing sand and diesel contents, providing a theoretical basis for long-term landfill remediation assessment.