If you are still looking for a German Siddur, the siddur of Rav. Hirsch is probably the best option. It is in the style of Minhag Frankfurt, which is more of the Ashkenaz minhag than Siddurs from Northern Germany.
The word הֲוָיָה is not an acronym or abbreviation! It's a word, meaning to bring into being - as in Hashem brings the entire world into being.
In the context of Hatarat Nedarim it refers to the ineffable name of Hashem - YHVH.
אם נשבעתי בהשם הֲוָיָה ברוך הוא
would, therefore, translate to
If I swore with the ineffable name of Hashem - YHVH - ...
The Chazon Ish (cited in Kol Nidrei, by R' Yitzchak Shtesman) says that it's because otherwise this declaration would cancel the vow to fast (unless he specifically remembers the condition at the time and is knowingly overriding it). The result would be that his fasts wouldn't be preceded by a proper kabbalah, which is needed for it to be a proper fast (...