Cellular vehicle-to-everything (V2X) networks leveraging device -to-device (D2D) communications face critical interference and reliability bottlenecks in safety-critical scenarios. This work introduces a dynamic resource orchestration framework that jointly optimizes transmission mode selection, spectrum sharing, and power allocation. By decomposing the mixed-integer non-convex optimization problem through block coordinate descent, our approach iteratively solves coupled subproblems. Non-convex constraints are transformed via successive convex approximation with first-order Taylor expansions, enabling efficient solution convergence. The proposed scheme maximizes vehicle-to-infrastructure (V2I) sum-rate while rigorously guaranteeing ultra-reliable low-latency requirements for vehicle-to-vehicle (V2V) links through adaptive mode switching between dedicated and reused spectrum access. Simulations confirm significant performance gains over conventional methods across diverse urban scenarios.