Even if they deleted each other's contact on their phone. I don't believe they don't remember each other's number . And they could obviously ask their friend or relative for the number .
No, it seems Xing Wu is not actually the son of her son. His mom got pregnant with Xing Wu with another man.
Yes, xing Wu mom was dating someone when she was young. She got pregnant but they didn't end up together. She married grandma's son. So xing Wu is not related to grandma by blood. But their relationship was nothing less.
Yeah fl deleting their contact was unnecessary , there were too many flashbacks in the last 2-3 episodes. But overall a nice ending.