Uta is the furthest thing from a home-wrecker and all of those other things. And no rational person would suddenly think otherwise, just by changing Reiichi's character to a female -- as I think you're trying to say. She literally tries her hardest to keep her feelings to her self, to avoid making things awkward in the apartment, and only lets it out when Kaoru insists on getting to know her better. And yeah, she's such an ungrateful bitch that she moved back in with her mother, whom she hates, in order to no longer be a financial burden on Reiichi and Kaoru, despite both of them insisting it's no trouble at all, and being against her moving in with her cold mother, who abandoned her 5 years ago...
And Vinveli, it is unquestionably cheating. Risako loves Kaoru and is trying to get them separated. It's no misunderstanding. Reiichi texted Kaoru saying that he was going to be away (in another town) on business for several more days, then she sees him that same day, walking with Risako -- a previous partner of his, and the only one he's ever been shown to have an interest in. Chapter 30 raws make it look like he went out with Kaoru after the death of her mother, which his mother was involved with, out of pity and/or duty. And it certainly fits with all of his interactions with Kaoru as a husband. Then there's the fact that Risako claims she hasn't seen Reiichi since their wedding, when there would be no reason to lie if they did meet up for such an innocent reason. Risako would also be the last person to ask for advice about relationships, when everyone who knows anything about her knows that she's barely capable of understanding emotion. Not to mention Reiichi clearly wouldn't ask anyone about something like that because he's shown to be pretty emotionally distant, with no interest in closing the gap.