I think the title will come first, Elba being plain Elba or Island of Elba.
Prince seems natural to me due to its usage in English and French where it's a courtesy title for members of royalty. I think German nobility were adopting Prinz for that usage around this time too. I think Italian principe has similar multiple usage as English and French so that might work initially. Especially if he can be awarded that while Napoleon I is still alive. Later on there'd be discussion about whether he ranks as prince minor (below a Duke, roughly on par as margrave/marquis) and thus a Fürst or as prince major (above a Duke, roughly Grand Duke but more sovereign).