I tend to agree. The premise as stated is really not credible. I would also question why the officer has left matters so late. Auschwitz, yes, perhaps a lack of subtlety. But Kelli does say its in concept stage, and whats the first place you think of when someone says 'concentration camp'?
Yes, holocaust fiction sells (if you wish to put it so crudely) but there is a distinction between fiction that is about the holocaust and fiction that seeks to use it as a handy backdrop. That you have chosen Auschwitz, that most iconic of death camps and the Waffen SS, that most loathsome part of the german army suggests a severe lack of subtlety.
I don't think it seems that incredible. If you start getting into the history surrounding the Nazi Party, and why they came to power, its conceivable that a more liberal minded person could be swept up by the politics and mob mentality, even if they weren't just doing it to protect themselves. Its about people, and people do strange things. Why would he leave it so late? Maybe he didn't realise the full extent of what Aushwitz entailed. Maybe he thought he could get her out before. Maybe he didn't realise how much he loved her until the last moment. A million different questiosn that probably need to be researched before a plot outline can really take shape.
God I love history