Oh, what the hell? Was it obvious? I didn't caught on the clues but whatever in the end that's one heavy plot twist. I did not expect that.
Also, she's pissed but more so is genuinely disgusted to the thought of ever meeting her biological father.
@lumpychicken basing on Mao's assumption, the baby-swap was later discovered and her foster father was punished because he failed to notice the swap. So some high ranking people should know of this. But I bet they don't know the identity of the crown prince even of they knew the swapping.