Czechia, Austria and Slovakia were part of the same country two centuries ago.
This sounds much more natural to my ear.
Yes, your English sentence is more elegant, but it is not a better translation of what the Czech sentence actually says. Right?
Depends on what you consider the best translation to be, one that conveys the sense best or one that keeps closest to the original wording.
Yes, I think it would be more historically proper and with Czech translation: Před dvěma stoletími byly Česko, Rakousko a Slovensko částmi jedné země.
Such a sentence does not fit this course.