It seems to me that the novel and the manga are slightly different. Novel Kayo seems to be an unrepentant whore that is only temporarily not seeing Iketani. But manga Kayo seems to actually have real feelings for MC. Manga Kayo tells MC she rejected Iketani's offer because she finally realized that MC was "the only one I truly needed."
The timing of it makes her words lack sincerity. If no such thing transpired (her being pregnant), I don't think Kayo would even bother apologizing nor give Yuusuke attention. Those words lack sincerity when you know there is a motive behind it.
If she said the same words before she discovered she's pregnant or become aware what Iketani had done, I would buy it.
If she cut off her ties with Iketani before she confessed to the MC, I would buy it.
If she didn't wait until the MC started ignoring her and said those words, I would buy it.
If she came open about her past relationship with Iketani when she started dating the MC, I would buy it.
If she didn't actively avoid the MC and still gave him the attention while she was cheating, I probably still would buy it.
Accepting Kayo's words as is has the same energy as watching Kayo shout "I only love Yuusuke, not Iketani!" to the whole world while she makes out with Iketani in front of MC.
Question is whether or not Kayo actually loved Iketani, or if she loved the excitement of being with Iketani? From your quote from the novel, it seems like even Kayo is not even sure, herself.
You have a point. She may be unsure about her feelings toward Iketani, not just MC. Her love for them might be both superficial for all we know. However, it doesn't matter because the fact that she paid more attention to Iketani means she favored him more in one way or another. After all, action speaks louder than words. It would've been difficult to justify if her actions weren't biased towards one side only. But as we've seen, her avoiding the MC was intentional and that says something. Regardless, whatever the answer to your question is, what Kayo feels towards Iketani, for the MC it is irrelevant. To MC, Kayo never paid him attention and he is nothing but a backup.