I would like to answer your questions as follows:
1) I am sorry to tell you that the pregnancy of your wife is not a good enough reason to end the contract.
When you close a rental contract for a certain time, § 542 Abs. 2 BGB says, that the contract only ends with the expiration of that time or if one party terminates the contract for an important reason.
On the one hand, this means that you cannot terminate the contract within the usuals 3 month deadline; this would only be possible, if the rental contract had no specific runtime.
On the other hand, the pregnancy of your wife is not a reason that is "important" in the sense of the law. § 543 Abs. 1 BGB says, that you have to weight up the intrests of the parties. In your case, the pregancy of the tenant or his wife is your own "risk". The lessor has nothing to do with that.
2) Your second question does not need an answer in your case. Anyway, it _would_ be sufficent if you wrote your own letter. It had to contain the reasons for the termination and you would have to sign it. But as I said, in this case you do not have the right to terminate the contract.
3) I also answered on question nr. 3 in point nr. 1. You do not have the right to terminate the contract within the 3 month deadline when you agreed to have a 1-year-contract.
The only solution i see is to talk to the lessor and tell him about your situation and that you feel the need to move out to a certain date. The lessor does not have to let you out of the contract, but he has the option to. He might connect it to some condition, e.g. that you have to present him new tendants. You might propose that yourself also if you think it will make him let you out.
I hope, the answer was helpful.
lawyer from Dortmund