Sora really is an idiot. Knowing full well what he's telling her would mean in regards to his feelings, he still does it anyways because they're both important to him. Well im ok with it as Im pro Mahiru
I didn't really get the feeling he was telling her not to give up, if that's what you're suggesting. Rather it sounds more like "whether you persevere or give up now, the last thing you should do is run away". Like how he compared it to inviting a friend to an activity they don't like: Asahi won't date her, but that doesn't mean she doesn't want to be with her. She loves Miya at least as much as Miya loves her, and she wouldn't be able to bear having to end the relationship they have. It's just she also knows that she doesn't feel that kind of love for her and so would not be able to make her happy in that way. If Miya ran, this would only make things more painful and awkward, and make it even harder to go back to friendship.
This kind of puts a better perspective on Sora's behavior. It looked like he was playing the standard "Dogged Nice Guy" trope, unwilling to give up on love even when he knows that his crush doesn't reciprocate. But maybe it would be more accurate to say that he's living the advice he gave her. He didn't stick around because he thought she might eventually change, and he's not telling her to do so either. But he'd rather remain a valued friend and ally with that dull ache deep down than become a bitter stranger who can't even talk to her.
Of course, there's also the question whether Miya's "I still have love in my heart" is referring to her feelings toward Asahi remaining despite the rejection, or instead is her recognizing what she'd said previously: that Sora too has become a romantic love.
PS, thank you so much for returning to this. The plot is a bit of a roller coaster but I love it and I'm glad to see translations return.