The guarantee is to allowing the reservation. As far as a car ending up not being there when booked, whether you have PC, 5* or no status, the situation is the same

It sounds like you have asked for reimbursement, the company has said no - you have been offered a voucher which seems to give more off of a rental than the taxi fare

You have raised it with customer services as an issue - maybe time to put it to bed - I don't think you are going to get much further with it