Instead of putting the whole code in the options (which will be stripped restarting mirc), let the options call an alias.
So first, fill "options > mouse > command to perform on double-click > nick list" with e.g.:
nicklistdoubleclick $$1
then add the alias-to-call in your remote, e.g.
alias nicklistdoubleclick { editbox -afp $+(08•,04,$1,08•) }