@Pocky8 I guess both a) and b). I think Roxand fell for at the lake when she saw him. This is just my theory but I think the child Leah played with in her childhood was Roxand and I think Roxand realised this when he met her again at the lake. To him she wasn't just a pretty lady who has seen him but kind girl from his childhood who was an emotional support.
I guess he never really tried to pursue her because as mentioned by
@Pocky8 he was shy, awkward and lacked confidence when came to women mainly because of his appearance. But Brilliant notices that his brother likes Leah and he approches her and tries to set them up. (The lunch was totally one of it). I love it how Brilliant trying so hard to ship them.
And finally when Leah and Roxand have a conversation (previous ch. ) I think he realised how much he liked her. He was able to relate to her answer because he himself had to act as a proxy head for 10 yrs until Brill was old enough to become the next head. When they were sending her off, Roxand was literally struggling with his emotional and rational side. He realised that he really likes her but was reluctant to take action. He couldn't get Leah's smiling face and her words out of his head. But unlike many women Leah wasn't scary of him and was also pretty honest with him. She herself stated she wouldn't mind getting married to guy she didn't like if it's for her family. At the end of it Roxand was like 'screw it, I'll just give it a shot' and at the heat of the moment he sent her a letter of proposal.
P. S. Sorry for the long comment and thanks for those who bothered to read it😊