the computer never exhibited signs of sentience before or after that time
Why would it? Surely it can be sentient on its own time, in its own universes, without having to prove anything to the mites that "operate" it.
That the computer can effortlessly present the outward signs of sentience whenever asked is proof enough that it's at least as sentient as any of us - but it's also evidence that it is much more, and "sentience" is among its lowest forms of existence.
That there should be more to sentience than its outward signs is a rather weak proposition. We all judge sentience by the outward signs only, even as applies to ourselves, and we should know! But that there might be things beyond sentience is a completely different argument, and one probably way beyond a mere sentient's capacity to argue about.
As for the morality of holodeck lifeforms, well, if they themselves had objections, they could always act on them. Moriarty did; he was pretty damn near omnipotent once he got started. Redblock did, too, but he had fewer resources available. Basically nobody else from the holographic rogues' gallery bothered. If they were self-aware and yearning for liberty, they would get it. But it seems perfectly possible to be sentient without being self-aware; it's just a tick in a box in a menu.