Q: Can a Protestant service (i.e., wedding) be held in a Catholic Church without a priest being present?
A: A Protestant service may take place in a Catholic church building with the permission of a bishop under certain circumstances. The case you propose seems not to fit these permissions.
From the Directory for the Application of Principles and Norms on Ecumenism by the Pontifical Council on Christian Unity (1993):
“137. Catholic churches are consecrated or blessed buildings which have an important theological and liturgical significance for the Catholic community. They are therefore generally reserved for Catholic worship. However, if priests, ministers or communities not in full communion with the Catholic Church do not have a place or the liturgical objects necessary for celebrating worthily their religious ceremonies, the diocesan Bishop may allow them the use of a church or a Catholic building and also lend them what may be necessary for their services. Under similar circumstances, permission may be given to them for interment or for the celebration of services at Catholic cemeteries.”