"This is bad"... i rather think is because Mizuki might not like it. If Hinata were to develop feelings for Hazuki too the story would completely become a mess.
I can only imagine this story having a disney magic kind of ending at this point (a happy end tho completely illogical). I mean, although is very clear that Mizuki and her husband dont really love each other it would be a huge mess going through separation mostly for Hazuki and worst of all if her mother ends with her best friend and beloved Hinata. Thats why Hazuki being in love kind of spoiled the only possible less worst solution i could imagine.