2)He thinks that Jan Jan was the person who loved him the most
Isn't that true? I mean a crush does not mean matured love, right? A love between a old couple is different from romance that occurs on a first date, or the first year in dating... So far, Siu Ling had never let herself to love Tin Yau more than as a friend, she suppresses her feeling by thinking that Tin Yau belongs to Jan Jan. On the other hand, Jan Jan has no reservation when she loves a person. She is always very clear about who she loves. After she knows that grandpa Tin Yau is not grandson Tin Yau, she understands that she should leave her feelings behind even though she misses Tin Yau a lot. After Szeto Fun Yan becomes a nicer guy, she is willing to tell him that it is possible for her to fall in love with him, if there's more time. Unlike Siu Ling and grandpa Tin Yau, Jan Jan is always very clear about who she loves. She might want to hug grandpa Tin Yau for 5 minutes for remembrance, but she knows how to let go of a person she truly loves (i.e. grandson Tin Yau). Hence, Jan Jan did love grandson Tin Yau more so than Siu Ling.
That's my view.