Two points:
On the “It didn’t sound like Risako was lying,” I just take that to mean “She sounded totally plausible even though I know she’s lying,” like Risako didn’t miss a beat when coming out with the lie.
(I must admit, I can lie when necessary, but when cornered like that I’d need a moment to gather my thoughts to make sure of what I was going to say and how I was going to say it.)
On the baby: Kaoru starts the day off in a daze, and two different people (manager and Risako) bring up having children. It’s not a big surprise that Kaoru would fall back on the (previously forgotten) long-term plan: what to do when there’s no Uta? Oh, that’s right—that’s when we were going to have a baby. Let’s have a baby.
https://dynasty-scans.com/chapters/my_unrequited_love_ch04#19
It’s obviously an astoundingly crappy idea, but given the limited possibilities and resources of the brain involved, an understandable one.