Were wedding gowns ever "passed down"?
Rarely. However according to "the grand duchesses" Grand Duchess Kira in 1938 at her wedding to Louis Ferdinand of Prussia wore her grandmother Grand Duchess Marie Alexandrovna's dress. It had originall been made for Marie's weeding in 1874 and had been kept in storage at Coburg for 80 years. No alterations of the dress was needed.
(wedding of Marie Alexandrovna in 1874 below)
I also wonder if her sister Maria wore the same dress when she married in 1925, though it's not stated in the book.
However, I've never heard of one of the Empresses wedding dresses even surviving though the coronation dresses, made to the same basic pattern can still be seen it the Kremlin. Marie's coronation dress is below: