Achieving high efficiency, long operational stability, and narrowband pure-red emission through rational molecular design remains a critical challenge for next-generation ultrahigh-definition displays. Here, we present an integrated materials-to-device strategy enabling narrowband, high-efficiency red multi-resonance thermally activated delayed fluorescence (MR-TADF) organic light-emitting diodes (OLEDs) with suppressed roll-off and extended operational lifetime. Three boron-nitrogen-oxygen (BNO)-fused emitters-4BPBNO, 3DBFBNO, and 2DBFBNO-were rationally designed by modulating multi-resonance pathways and incorporating dibenzofuran (DBF) units. The rigidified π-framework suppresses vibronically active C-C stretching modes, enabling precise color tuning while preserving MR character and high photoluminescence quantum yield (PLQY). As a result, 3DBFBNO exhibits red emission with a peak wavelength of 613 nm and a narrow full width at half maximum (FWHM) of 32 nm. To overcome the intrinsic roll-off of MR-TADF systems, an exciplex-assisted phosphorescent-sensitized fluorescence (Ex-PSF) architecture was employed. The 3DBFBNO optimized device achieved a peak external quantum efficiency of 31.6%, <1% roll-off at 5000 cd m, and LT of 571 h @ 5000 cd m, representing a 13-fold lifetime enhancement over single-host devices. Additionally, an optical-cavity-engineered top-emission OLED incorporating 3DBFBNO further achieved 619 nm emission, 23 nm FWHM, along with CIE coordinates of (0.68, 0.32), approaching BT.2020 requirements.