Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains a significant global health threat because of its rapid evolution and high mutation rate, which limits the performance of existing molecular diagnostics. This study presents a dual-mode, aptamer-based detection platform that combines high sensitivity with mutation resilience. Using a computer-assisted X-aptamer Systematic Evolution of Ligands by EXponential enrichment (SELEX) approach, we identified NP14, a high-affinity, dual-target DNA aptamer that specifically binds to the SARS-CoV-2 nucleocapsid (N) protein at its N-terminal domain. Analyses via molecular docking, aptamer truncation, and targeted mutagenesis revealed that NP14 interacted with both SARS-CoV-2 and SARS-CoV N proteins and identified key nucleotides C24 and G27 of the P1 region and structural determinants critical for its high-affinity binding. Building on this discovery, we engineered a dual-mode biosensing system by integrating NP14 into a multicolor dynamic light scattering-enhanced enzyme-linked aptamer-antibody assay (MD ELAAA). MD ELAAA synergistically combines two complementary detection strategies: i) non-aggregative plasmonic colorimetry for visual signal detection and ii) dynamic light scattering for ultrasensitive quantitative analysis, in which Au/Ag nanomaterials are used to amplify optical and scattering signals. This system achieves a sensitivity of 0.43 TCID50/mL, representing a 47-fold improvement over standard methods. By integrating high sensitivity, specificity, variant recognition, and dual-mode signal output, the MD ELAAA platform enables reliable detection of low-abundance SARS-CoV-2 antigens. Its robust performance supports early-stage diagnostics and high-throughput variant monitoring, establishing MD ELAAA as a robust platform for next-generation viral detection and surveillance.