We derive tighter off-policy bounds for LLM-RL: O(T^1.5) Pinsker-Marginal and O(T) Mixed bounds, compared to classical O(T²). We propose Trust Region Masking (TRM), which excludes entire sequences from gradient computation if any token violates the trust region. TRM is the principled form of masked importance sampling (MIS): a sequence-level trust-region mask combined with per-token importance sampling.