Is the suitor entitled to retrieve all expenses and gifts paid after the dissolution of the engagement, given that he completed the engagement and part of the preparations?

As long as the suitor has not entered into a valid Islamic marriage contract with your sister, he has the right to demand the return of the dowry gold (al-shabakah), and your sister has no right to it. As for other gifts apart from the dowry, there is a difference of opinion among scholars regarding the permissibility of reclaiming them. The more cautious approach is to return his gifts to him.