Can I use an existing policy but modify it per traveler?
Yes, you can select an existing policy. You then will have the option to update dates and form of payment.
How can we avoid the difficult reimbursement process for candidates?
You can specify that all travel must be paid for using your company card. Then the traveler will not incur any costs associated with the booking.
How can we ensure that the candidate does not book travel elsewhere?
You can specify the travel destination to prevent candidates from booking travel to the wrong city (to prevent mistakes as well as malicious bookings).