Rosamund is a beautiful name!
I don't like shortening of names, I don't let anyone shorten my name. I have two DDs and I didn't let anyone shorten their names either.
When they were of age to use whatever version of their names they preferred, DD1 refuses to let anyone shorten her name, and DD2 has a four letter name that can't be shortened anyway!
So, I think until your DD reaches that age, and you want her to be called by her full name and not a shortened version, you just tell people her name is Rosamund, not Rose or Rosie.