Can't help but think these kids need jobs. If they put as much effort into finding one as they did learning this secret menu stuff they'd have enough money for that not to matter.
You can deduce that the one she loves is the bondie Yohei. She sits next to him at the diner, she has more commonalities with him like knowing the secret menu of the diner, she only notices small hints that dark-haired Sato liked her but completely was surprised by Yohei liking her, and if you see the positioning of the dialogue boxes at 15 Sato is the one who believes he won't be picked. Even when she returns in 18 she immediately returns to stand by Yohei even after all the confessing just happened.
They both probably know that she likes blondie, and are just riling her up to get her to confess to him. In a "bro" kind of way, they're working together to help her pick. Like an honorable duel between brothers-in-arms to receive the hand of the duke's first daughter, when both brothers know with certainty that the daughter loves one of them.