Runaway electrons (REs) pose a serious threat to in-vessel components in high-current tokamaks like ITER, particularly if localized RE beam energy deposition occurs following a plasma disruption. Their reliable detection and control are therefore high-priority research areas. This work presents joint simulation and experimental studies using a prototype of the ITER Hard X-ray Monitor (HXRM), recently validated on the ADITYA-Upgrade tokamak. The prototype system, designed to operate in extreme ITER-like conditions, decouples the scintillator from the photomultiplier tube and uses relay optics for enhanced survivability and remote readout. Comparative measurements using both conventional and ITER-prototype HXRM configurations demonstrate good agreement in time and energy-resolved HXR signals for various plasma discharges. The dual-mode detection (counting and current modes) scheme of the HXRM system enabled effective monitoring over a wide dynamic range of RE parameters. Sensitivity to plasma parameters namely density, gas puffing, and MHD activity was demonstrated, with the evolution of HXR-spectra captured through pulse height analysis. Preliminary RE simulations using the PREDICT code show a reasonable agreement with experimental observations, validating both the diagnostic approach and simulation assumptions. These results represent a critical step toward ITER-relevant RE diagnostics and enhanced confidence in runaway electron modeling approaches.