I've heard that Emirates shifted their deposits from the A350 to the 50-frame A380 order they placed last year. They have yet to choose an engine supplier for those frames, so that may signal a switch to RR power for them (EA is their current A380 engine supplier) and Tim Clark has stated in the press he believes only RR will offer a "new engine option" for the A380.
Early A350-900s are heavy, but not terribly so and Airbus is reducing the weight much quicker than Boeing did on the 787-8 so I don't think that clause (if true) was the basis for this cancellation.
I think EK could add some more 777-300ERs in the interim, but they do have 50 options on top of the 150 777X commitments, which would cover the A350 on a total seat-capacity basis. And EK could always extend their current 777-300ER leases to cover the time-gap to the 777X.