1. Kantians believe that we need to follow the maxim when we do thing. Even though we lie, and it seems that we violate the principle of humanity because if we lie to Nazi officer, they use this officer as a mean. However, if we consider the maxim. Our maxim is that: whatever I need to say in order to prevent the murder of an innocent person. There is no absolute moral duty for us unless we consider the maxim first.
2. It’s hard to say that this strategy consistent with the formula of humanity because the we lie to the officer and use him as a tool. However, we also save humans’ lives. We also follow the formula of humanity to some extent.
3. The practice danger is that the officer may debunk our lie, and he will still kill the family and I for our lie. Therefore, by lying, we still not get the result or maxim that we want.