I think I might see what Nikaido's character motivation is. She definitely has feelings for the MC to some degree, but then in swoops the most high-spec of rivals to snatch him away. And really, why would someone of Hitotsuba's stature want to be with someone who is average/ordinary by comparison? Maybe she's stringing him along, following some wild whim, only to eventually dump him when the luster wears off and she gets bored.
The MC may also feel the same way to some extent, especially when taking into consideration that their relationship begins with him at his lowest, abandoned by his family, and the two would-be lovers/future spouses effectively being strangers. Seriously, out of all the fish in her sizeable pond, why the hell would the most admired, sought-after, and incredibly affluent JK in Japan fall for and want to live with him right out of the gate?! What's more, even if she is being one-hundred percent sincere, the beginning of the manga still portrayed their introduction as akin to a rich person buying a pet. Hell, I'm not a huge fan of arranged marriages, but I think even that would have been more dignified than what we got.
Finally, why would the MC prefer a lunch that reminds him of the parents that abandoned him? The answer, I think, is memories. The MC is recalling the simple, happy days of his childhood when his immediate family was still whole, and his days were filled with warmth, love, and his mother's homemade cooking. Nostalgia is a powerful thing, and it wouldn't have been out of place even in a gag/rom-com manga like this for the MC to tear-up at that moment. The guy has been through something rather traumatic, which can be easy to forget with the tone and his seemingly stoic demeanor so far. To be honest, I kind of wish he had shed a few tears then and there. It could have served to make the girls realize that in their selfishness and jealousy, they are overlooking his suffering and feelings towards the whole situation (even if Nikaido and the rest of his classmates don't know the full story about him living with Hitotsuba).