Quantum computing erodes the security foundation of classical public−key systems, prompting global financial market infrastructures (FMIs) to accelerate their migration to post−quantum cryptography (PQC). The Phase 2 empirical tests of the Bank for International Settlements (BIS) Project Leap reveal the engineering constraints of shifting real−time gross settlement (RTGS) systems to hybrid cryptography: native schemes do not support parallel processing of heterogeneous algorithms, and end−to−end PQC signature verification takes about 209.9 ms, which is 7.5 times the 28.1 ms of classical cryptography. Our analysis shows that this overhead stems mainly from system−level stages such as message parsing and verification−software orchestration rather than from the lattice operations themselves; placing it on the synchronous critical path of high−concurrency settlement would depress throughput. This study therefore decouples the processing chain so that the upgrade can proceed in stages: a nested signature with an inner PQC layer and an outer RSA layer is constructed in the Business Application Header (BAH), allowing legacy gateways to accept messages without parsing PQC extension blocks (zero−intrusion access); the verification latency is offset by the asynchronous rollback of a distributed transaction, in which a pre−settlement hold removes synchronous blocking and a compensating transaction restores global consistency, overlaid with a quantified monitoring model featuring anomaly−triggered circuit breaking. Simulations show that, once verification is moved off the critical path, the settlement−decision latency drops to the millisecond level and the software−only path already meets the conventional 500 ms monitoring ceiling; with an optional FPGA pipeline accelerator to suppress jitter, the threshold can converge to within 50 ms. The analysis of a three−stage evolution path further supports this conclusion: with proper decoupling and sound asynchronous orchestration, real−time throughput need not be sacrificed, and financial core systems can steadily complete their quantum−resistant security upgrade.