Lovely cup

He fell for one person and it's actually fengyin. The waterbeast egg was already broken (has no soul) a long time ago and it just happened that Feng yin's soul make it alive. She still let her close friend call her "Ayin" later when she become phoenix emperor. Ayin and fengyin personality might be completely different due to the reincarnations she's been through for a thousand years. In the book, feng ran (feng yuan/the current phoenix emperor in drama ver) said that she noticed a change in fengyin's personality (fengyin didn't tell her about what happened to her a thousand year ago after her soul scattered). Remember when she first born in ep 1? She was an adorable and curious little phoenix, just perfectly like ayin.

Yuanqi found out the truth about hongyi controlled by the demon spirit first way before ayin, he has a plan to lighten up the punishment for him, but he did not tell ayin, so after ayin found out she rushed to save hongyi (which was against the heavenly rules), yuanqi tried the best way to save ayin but unfortunately failed bcs he didn't know huashu illegally punished ayin before him. The only stupid thing yuanqi do is he doesn't tell ayin about his plan, just completely the same mistake his dad done in the past

Yuanqi will find out that the person who helped him and he's been adoring for a long time is actually fengyin, he will realize that he was with fengyin's soul all the time

Yes, later in the book, feng yin explained what happened to a water beast ayin to hong yi when they first met after she become phoenix emperor. She said that one of her scattered soul fell into a demaged water beast egg, gujin used the drunken jade dew to nourish its soul, but it accidentally awakened her phoenix (ep 1-2), so basically ayin is feng yin all along, she didn't replace or take any other's soul that's why her memories of ayin stay until she wake up as the phoenix emperor