Re the flags. I suspect that the reason may be that there were already the French tricolor and the Dutch red white and blue flags. Adding the USSR flag may have been more immediately clear than adding a third red white and blue variation. And they're in modern dress anyway.
If I were nit-picky as well, I would fault with the simplified kanji (Chinese Character) used in the treaties written on white bands during Someone in a tree, which wasn't adopted until a few years after WWII.
I wasn't fond of the pictures and modern dress either, but in person, it's extremely intimate and the staging is quite lovely to watch unfold on the unique set.