this series is great because it shows very realistic relationship issues in a way that the majority of other romance mangas do not. i think miwa choosing to not make the easier decision and fall into old patterns is a big moment for her. she needs to realize that being given the opportunity to cheat and not doing it is not a betrayal of saeko's trust just because she didnt have the confidence to say that she was in a relationship. hopefully she realizes after being put in the awkward position of exchanging numbers with her ex that you need to be willing to sacrifice some of your own comfort to protect your relationship. maybe she wont, who knows? thats the reason we all keep reading, isnt it?
and i wouldnt think too much about the per-serialization too much, there's very little that carried over other than Saeko's personality and the character designs. Miwa is a completely different person in this.