It's interesting that MC was in such a poor position in her own household, but clearly knows how to be manipulative and gain power - I wonder why she didn't seem to take any action for herself at home then? Perhaps she didn't care much for acting in her own best interest, but is willing to do so for the princess she serves?
Or maybe her family really cut off every avenue of gathering power that she had?
Isolation was probably a major factor, but I think she just didn't care about position and royalty during her youth. Considering she was all "soon I am 18 and then it is over" thoughts before prince-ex-machina wiped the board clean, it is quite likely she just wanted to escape her family. Maybe she already had a plan to get married off, but disappear mid journey or something. Her willingness to become a maid at the start kind of reflects she had no intention to stay an aristocrat and had planned for something similar.
Hard to say and guess unless the author decides to revisit those times. Biggest factor though is, that this time she isn't doing it for herself.
Will be interesting to see how all this plays out, my bet is that she had the first letter written so that it would get ripped/enrage the dowager. Most likely, the old queen will soon pay the estate a visit/summon them for a lesson/chastising or something only then to be converted. Mayhap even have the dowager view the MC in a bad light and think of her as a manipulator and thus save the princess from her clutches or something.