Quantitative water analysis in methanol-rich solutions represents analytical challenges in terms of broad, band-overlapping, and concentration-dependent near-infrared behavior of hydrogen bonding liquids and the nature of near-interface sampling by fiber-tip plasmon resonances. This study proposes a range-resolved approach for the quantitative interpretation of methanol–water samples based on combining two near-infrared absorbance regions with localized surface plasmon resonance tuning of a nanostructured optical fiber sensor. The main research problem is whether water fraction from 0 to 50% wt can be better analyzed when divided into different sensitivity ranges instead of integrating all three signals, such as absorbance at 1450 nm, 1950 nm, and plasmon red-shift wavelength, in a common calibration curve. Concentration values are organized into a ten-level composition series, which then are expressed through four analytical descriptors: normalized band area completion, signal increments at the first calibration range, band area ratio, and accumulated plasmon blue-shift value. As seen from the descriptor table, the contribution to absorption in the combination region at 1950 nm rises to 52.9% upon reaching 10 % water content, whereas absorption in the overtone region at 1450 nm reaches just 35.3% within this range. The LSPR center shifts from 1527.0 nm to 1504.2 nm, providing a total blue shift of 22.8 nm. The data confirms the effectiveness of using a range-resolved fusion scheme in this case. The reason is that water quantification in the lower ranges of interest is achieved due to the 1950 nm band, while the high range continuity is ensured through the overtone band at 1450 nm. Moreover, the plasmonic response provides independent verification of the interface conditions. Thus, the present analytical scheme is a useful tool for analyzing methanol-rich solutions in terms of their liquid chemistry and instrumentation specifics.