Yes, they are both WebKit, though largely irrelevant. Personally, I'm a daily KDE user. I did install falkon out of Curiosity for a short while. But neither it or konqueror are generally in any distributions base install. Including KDE neon.
They are usable, so long as you don't use any sort of add-ons.
Falkon is based on Qt web. So it is also WebKit.
Correction Falkon and Qt web engine are indeed chromium. Konqueror alone then would be WebKit