As for "when does the manga take place historically?", the manga references the "collapse of the Habsburg dynasty" 100 years earlier. Because the history of the dynasty is so long with so many ups and downs, it's hard to be perfectly sure what the author is referencing here. My best guess is that they are talking about the dissolution of the Austrian Empire in 1867, which was sometimes called the Habsburg Realm. Additionally, the author refers to the country as "Austria", which places the setting either after Austria-Hungary split in 1918 or 400 years earlier (and that seems very unlikely because there would be almost 0 black servants in Austria in ~1500).
With both of those clues together, I think the story's setting is the "present day" (for the author, ofc), meaning 1973. This is 106 years post-collapse, so the math checks out for "100 years ago".