I see you linked an old post of mine, there are other ways to read/write to ini files.
Maybe this is of help, since you are developing your own plugin you might have enough freedom to use the method in my reply here:
How to select on which monitor to display the game ? - #10 by Roy_Wierer.Seda145
In case the link dies, a small example on GConfig:
GConfig->SetString(*Section, *Setting, *Value, GGameUserSettingsIni);
GConfig->Flush(true, GGameUserSettingsIni);
I have never seen this with my methods and I don’t know what ini editing method in specific and engine version you use.