Maybe your question is answered here already:
Yes, you can. As German citizen, you need to do a name declaration according to German law, even if you are already using your new name abroad. Please find more information here:
Yes, after dissolution of a marriage or registered same-sex partnership, you can return to your former or your maiden name. However, please be informed that your divorce, if decreed abroad, might not be recognized in Germany yet. In this case, you have to apply for recognition of divorce before you can lodge your name change application.
No. Since you never changed your name, you don't need a name declaration now.
Yes. You can keep your married name/ partnership name, even after it was dissolved.
No. Germany does not recognize "de-facto" relationsships. You can only change your name after a marriage or entering a registered same-sex partnership.
It depends on the citizenship of the partners. If one of the partners is a citizen of a country which allows double names (e.g. Australia), you can take on a double name through name declaration. If both partners are German citizens, you cannot take on a "real" double name - however, if you take on your partner's name, you can add your former or maiden name to the married name. Nevertheless, any children born into the marriage/ partnership, will only have the married name as their surname, but not the double name.
For more information, please read here: