A key difference is support lifespan, though: 5 years of standard security maintenance for regular Ubuntu[1], and 3 years for Kubuntu[2].
[1] https://ubuntu.com/about/release-cycle
[2] https://wiki.ubuntu.com/NobleNumbat/ReleaseNotes/Kubuntu
Yes though you can easily install kde on normal Ubuntu.
Yes though you can easily install kde on normal Ubuntu.