Visual Quality Assessment (QA) in multimodal large language models (MLLMs) jointly predicts absolute quality scores and relative perceptual rankings. Existing methods optimize score regression or ranking within a unified formulation, leading to unstable predictions and weak generalization under stochastic generation. We find this instability is amplified by response variability in MLLM reasoning, where stochastic decoding produces inconsistent score distributions and degrades cross-dataset robustness. This indicates QA depends not only on preference alignment, but also on response-level stability. We propose PreResQ-R1, a Response-Preference Disentangled Ranking-and-Scoring Reinforcement Optimization framework. It decouples response stabilization and preference learning, with a dual-branch reward modeling intra-sample consistency and inter-sample score-ranking alignment. For video QA, we introduce temporal-spatial evidence aggregation without dense motion modeling. PreResQ-R1 achieves state-of-the-art performance, outperforming comparable methods by 5.60% on IQA benchmarks and 2.53% on VQA benchmarks.