Reusing the Rate-Splitting Multiple Access (RSMA) common stream for sensing eliminates dedicated probing signals in Integrated Sensing and Communication (ISAC), but creates a feasibility bottleneck: the common-power fraction must simultaneously support common decoding, private decoding, and target detection. This paper investigates this tradeoff in downlink zero-forcing RSMA with imperfect channel state information at the transmitter (CSIT) and imperfect successive interference cancellation (SIC). With the common beam fixed over a sensing dwell through sensing geometry, the common-power fraction becomes the sole design variable. We prove that the communication-feasible set is either a closed interval or empty. Analytical outage expressions localize these endpoints, revealing a high-signal-to-noise ratio (SNR) floor-ceiling effect: common decoding dictates a common-power floor, while imperfect CSIT and SIC impose a private-decoding ceiling. For sensing, the conditional detection constraint is affine, preserving the interval structure. A geometry-matched common beam ensures monotonic detection improvement, reducing joint-feasibility check to the communication-feasible upper endpoint. Simulations validate our analysis, identifying regimes where common-stream-only sensing is viable.

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