Ruled alone? Who would have done anything different other than talk smart in hindsight?
Did not rule alone, however, the Black Zero as a holy grail was brought forward by the CDU, and the relevant ministry was predominantly led by the CDU.
Who did it differently (not would have done), I have already answered exemplarily with Austria. You will not be alone. The USA, as another example, have been holding bonds for far too long.
Yes, the 2017 one is still okay. But later ones also had 0.85%. And don't forget, the higher yield now comes because you can buy the 2017 one at 75% of nominal value. Whoever initially invested back then suffered a 25% loss with one bond in 5 years. The 0.85% one is now at 44% nominal value. Long-term bonds are not an investment for people with a need for security.
That depends on the purpose of use. It is important not to misunderstand the bond as a stock. Certainly, it is not a financial instrument for everyone. However, the target group exists, and the state would have been wise to do as its neighbors did. But special paths are 'our' thing.