But Spanners,
The 8 weekly is not being touted as all A350.
There seems to be several sources to indicate 4 weekly B77W (obviously already operating so accounted for in the allocation numbers), and 4 weekly A350, to which 3 frames should be enough to run AKL and MAN 4 weekly each to which the rumours suggest will happen.
Therefore, 8 weekly with the mix of the 2 aircraft as suggested is in fact possible within the timeframe quoted.