Like with robbery and rape, the frequency of murder in kidnappings makes it a plausible and unknown threat against your life, justifying the preemptive use of lethal force.
IF this stands, then I think there's no question of abuse situation being valid as self-defense. There is a clear plausible and unknown threat against your life and of bodily harm, and even though it is not "imminent" it could happen at any moment and is both inevitable and unavoidable in most situations
And lastly you'll note I use the term false imprisionment rather than kidnapping. Legaly speaking the only difference between some store security gaurds holding you longer than they're supposed without informing the police and a kidnapper locking you in his basement is the threat of immediate harm that getting locked in a basement carries with it.
If that is enough of a justification for a "threat of immediate harm", I don't see why this wouldn't be. We all know the outcome is going to be at best significant physical emotional damage and is all too likely to result in death. Picked my example because it was actually analogous - because I wanted to know whether the fact that pain or death will obviously be coming at /some/ point, and there's no other way out, would that be enough justification for self defense even if the threat isn't /immediate/ but could happen days or weeks from now. There's an implicit (if not explicit) promise of harm in the detention.
If it's obvious that they are not only making it so you can't escape, but ARE going to inflict gross physical harm on you at some point in the future, would self-defense apply, even if we don't know exactly when the physical harm would come?