It's not my call centre - although you are quite right Manx Crab that you shouldnt have to ask for a Manager everytime you call but it could be that it was a new member of staff.... I really dont know. What I would do if you come across it in the future is ask for the Manager rather than grumble about it on an online forum (not suggesting you have personally but others have - people allegedly working in the aviation sector).
Having said all that hopefully that wont be the case (excuse the pun) and soon you will be able to get things booked as you wish without the need to make any calls at all
If you wish to book singles in the meantime thats fine, but just bear in mind that they wouldnt be legally obliged as far as I know to change your return flight free if your outward flight was disrupted something which at the moment is rather beneficial! So there are pro's and con's of flying on two single tickets rather than a return