There was also Gerlach and Georgine(and prob anyone else who was at that meeting) Also a while not truly noble Egmont also knew. Also practically confirmed that the noble behind Wolf knew but we don't know who that was. Highly likely that many others suspected but couldn't get confirmation one way or another so it's just better safe then sorry especially when she was an ADC. Trudeliede also prob knew she wasn't Karstedts but prob not where she was actually from.
a lot of the former veronica faction and georgine's faction have been told that Rozemyne is a commoner former blue shrine maiden, but it's hard to understand the extent to which they believe it. Hartmut figures it out on his own as well.