After some thought I think I've figured out why I dislike the ending; I had trouble with the notion that Hanako would suggest surrogacy, and then follow it up with "oh by the way, I totally betrayed your trust in a moment of extreme selfishness." Quite apart from questions of whether that approach is even in character, it just doesn't seem like a good way to present things. I can't imagine she'd think it'd work out well for her.
I do, however, like the idea that Lilly and Hisao talked about it sooner rather than later. I think if it were me I'd have Hanako suggest the surrogacy idea, and then have Lilly bring up the kiss in response. The question would be "wait, why are you really suggesting this? Is it just guilt?" Cue hysterics and confessions and apologies from Hanako. After everyone realizes Hanako's suggestion was genuine, and reaffirm the depth of their feelings for one another, they can maybe move on toward a poly relationship from there (I don't think they'd jump in to it quite that fast, but you never know -- these sorts of relationships
do happen in real life, after all, and the framework for one is there in the relationships established both in canon and earlier in the story).
In other words, the end result wasn't bad. It was just a bit abrupt and reached via brute force rather than storytelling finesse. Happily, this is fixable.
And themocaw, get to it! You opened the door, now you gotta step through!