I don't understand people who think the ex is an option. The guy seems completely over Keiko and even when he was whining over her he didn't seems to try to recontact her or do anything... I don't see how it can be good for her to go back to him, the guy just gave up.
Crushes work in mysterious ways. Just because the only thing he does now is whining doesn't mean he's given up on her at all. I don't think anyone sees how it can be good for Keiko to go back to him, but that's not necessarily the case for her herself. If she really had true feelings for him, and if he really tries his best, in her opinion, like, you know, melts her heart - it may happen and she may forgive him. At least they don't seem like they properly ended their relationship, as it felt more like Keiko ragequitting due-to the miscarriage.
That essentially doesn't mean it's a het ending either, of course, because they are likely to break up very soon again just as well, forever this time. Or Keiko may reject him from the get-go. Or the guy might not even get the courage to get close to her again. It all depends on how string will Shou decide to troll the audience. Because, come on, essentially the manga is about the two girls living and stretching together. So everything that breaks this flow for more than a chapter or two will break the manga itself.
In that sense, I agree with you that the guy isn't a real option for Keiko. It's just that I'm afraid she's blind to it yet. And I wouldn't be that confident about the ex approaching Keiko again. Even if he tries to go for Ran, once he learns that Ran and that Keiko are roommates, he might reconsider and try to approach Keiko.
Cynicism is the only way to save your soul.
amen.