I can't test this but a possible nudge in the right direction could be
private IQueryable<Contact> GetContacts()
{
var people = _contextProvider.Context.Contacts
.OfType<Person>()
.Include("Employer");
var organizations = _contextProvider.Context.Contacts
.OfType<Organization>()
.Include("Employees");
return people.Concat<Contact>(organizations);
}