It was sent by the prince I believe (unsure though)
Flashback one from when her husband was alive, not the new one.
I believe it's the king. The Marquis was in love with the previous Queen, who died after giving birth to the prince. He married Shuri because she looks like the Queen and when the king saw Shuri, he probably also felt she looked like the Queen and sent to necklace hoping to be her lover. The Marquis was so angry because it's an exact repeat of before (not an excuse to hit your wife but that's the implication.
Who gave that necklace to her anyway?