You might be right in MSN6 being a bit late. In Airbus's press release for MSN2, it claims that MSN6 will now be delivered in Q4 instead of 2H. Whilst this is not exactly a slip as 2H includes Q4 it means that the aircraft is unlikely to be delivered in Q3 although I'm open to surprises!
on Thursday 2nd January 2014 Airbus rolled-out its third A350 XWB flight-test aircraft, MSN2, from the paint shop in Toulouse, marking yet another successful milestone on the path to entry-into-service in Q4 2014
I wonder what is setting the pace of the first A350 deliveries : is it type certification or the production/testing timeline ? I hope it is certification, as in that case Airbus might be able to deliver several A350s this year.
I doubt MSN 6 could be rolled out in June, but if it was, it could be tested on the ground and in flight even before type certification is granted by EASA to Airbus. Delivery could therefore follow certification, after customer acceptance flights.
Likewise, production pace being one frame per month, a few other frames could then be rolled out, tested, and delivered before year-end.