@gomichandesu
One of the beautiful things about this series is the difficulty they have working through the marriages. It's not a simple there you go, their all bonded to each other like in most Fantasy stories.
Those are nice, but they lose some of the realism... and while we don't often read manga (or comics) for their realism, it helps at least occassionally for a breath of fresh air.
Last I recalled, Koharu was the 'favoured' wife, the problem is she's also the most junior wife, #3. While I want to see Madoka come back, it'll be troublesome and a new arc of drama for Koharu to deal with.
Keeping in mind the story for the most part is seen from Koharu's POV, and she got thrown into it to save her family from debt/cafe. I'm sure it can be a huge struggle to maintain something that she railed against (men cheating) so here we are, with the character Koharu still dealing with being the non-primary relationship, or at least I think thats how she sees it even if he TRIES to make it even.
The other problem is she has yet to have a child with her husband... and with the time jump, I'm sure that weighs on her as well.