I think that Erynn does/did love the MC, but back then it was more of a sibling love for her. There are a few times in the game where I question that though. During the confrontation with Lexi, Erynn says "But you know how I feel about him!" "You know how much I missed him and how happy I am that he's back" and implies that only Lexi knows her feelings about the MC.
I agree that the 5 year break was a positive for their relationship to take a romantic turn. As hard as it was for them, it was necessary.
The 5 year break allows her to see him in a different light now that he's grown up, had they kept being neighbors it may have been harder for her to see him in this fashion.
I think even though she thought of him as a brother, that declaration in the backyard stuck with her(and she probably talked about with Alexis, before and after the MC coming back into town, so Alexis knows).
They talked about things since Erynn also knows about the stolen kiss.
You're right, it was more of a sibling type love, but as the saying goes "Absence makes the heart grow fonder".
I don't think only Lexi knows how she feels though, Mel probably does and I suspect Ethan too, which is why he's antagonising MC after returning. Erynn never cared about him as much as she did MC, so he starts manipulating Zach to betray his oldest friend.
Melissa knows even if Erynn never told her anything, simply mother intuition, she keeps pushing and teasing for them to get together, so she believes this is what her daughter wants, even if she doesn't vocalize it.
Ethan disliked the MC since he was a little boy, saw him as constantly getting in the way of his plans of conquering Erynn... and probably knows that the adult MC presents twice the problem.
