Mango prebooked seat problem

I booked on line a one way ticket for 2 passengers and included the extra at a cost of R55.00 each to allocate seats up front. The purpose of this was because I was unable to check in until a few hours before departure as I was coming in on an International flight and was going to travel the JHB to Durban leg with my granddaughter who is 8 and wanted to ensure that we could sit together. I booked seats 3A and 3B paid for the privileged and assumed all was well. About 5 hours before departure time I managed to go on line to check in and saw that our seat numbers had changed to completely different seats. I tried to phone the call centre and was put on a waiting list for over 15mins before I put the phone down and dialled again, once again put on hold for over 6 mins before I once again put down the phone. Very expensive for a cell phone to communicate this way. At no stage through the waiting was I ever told what number I was before service, just a message saying due to high call volumes please hold. I then sent a urgent email to *** explaining my problem and got a auto reply saying that they would deal with the content within 48 hours. I then got to the airport and on book in told the agent that I had paid for the privilege of early seat reservation and wanted those seats. She said no problem and issued me with my 2 boarding passes at seats 3A and 3B and I assumed all was good and back on track. When we went to board - after the flight was delayed by nearly 2 hours and got to the boarding gates the check in agent said that I had already boarded. I explained my situation and after being told to stand aside someone went off and came back about 5 mins later and said the problem was sorted and to board the plane. When I got to the plane there were 2 people sitting in my seats, so I spoke to the hostess and despite being told it was sorted no one knew about it. She went and spoke to the 2 passengers who refused to move. All this time I and my granddaughter were told to wait in the serving hatch. Finally they had to call for security to get them to move and we were seated and the flight started. A really bad and unnecessary experience and how does something like this happen? Still no feedback fro the email. Going to be difficult to choose this airline again.