"How likely is it that down the line we’ll change our chosen engine or develop our own? "
Their ansewer: would depends of the state of future engine and integration:
"The short answer is that we depend on the many modifications we have made to the Chromium code, and 'translating' these to a new engine would not make much sense ... *at the moment.*"
So what I said, they'd see. https://vivaldi.com/blog/vivaldi-different-from-chrome/