Parables, from what I can see, are each meant to make a single, specific point, in a manner that almost anyone could understand it, and that's it. They are not meant to dissect in fine detail for doctrine- except possibly for the single, specific point. The parable in question is rather pointedly about forgiveness.
So, in the parable, the framing story shows a person in prison until a debt is paid. As a basis for doctrine, that's missing the mark (to put it nicely.) Shame on JS if he couldn't just see that immediately, let alone catch it on a later read. As I see it, for him to miss something that obvious means he didn't WANT to see it, and was busy trying to justify something he wanted to see, even if he had to torture the verses to PRETEND that's what they said.
Right now, it makes no sense to me for a punishment to be more suffering and THEN annihilation. I'll have to look over the 9 verses and see if, somehow, it makes sense to me afterwards.