They all wear tiaras on state and major occasions. That's why the Grosvenors had one for Olivia to wear. That is why the Spencers had one for Diana. There was a whole thread on tiaras last year.
I think Olivia looks lovely and happy. Her tiara and flowers are from her husband's family. The lace on her veil is from her family. I suspect that there was a story behind the shoes, other than their color. Everything was chosen for its meaning, not just for aesthetics.
The bridesmaids' shoes looked very comfortable. I don't know how active the reception was, but black shoes show much less wear for children. Those girls may have been on their feet for much of the day.
It's nice seeing a happy wedding. I wish them all the best.