So how long has it been since Er and her father/grandmother all reunited? It sounds like only a year has passed since her father is allowed to all reunite with them for the grandmother's birthday?

Sorry, I'm not familiar with c-drama or historical dramas so if someone could explain this to me (hopefully without spoilers involved) that'd be great.

Not sure about the timings. Must be around 6 mnths to 1.5 years. Maybe more. we don't know about the timings.