Bringing Narcissa to the island wouldn't be terribly difficult. Her release date is at the end of the week and the Aurors will escort her to the manor in Wiltshire where Hermione could kidnap her. Ronald was a bit trickier, but the red head was an idiot more than half of the time. All she had to do was ask him to meet her somewhere without telling anyone else, and he would. He trusted as easily as his Jack Russell patronus suggested.