I'm not sure why it would have been so difficult to push merlins into the whirlwind. The merlin weighed about 200lb more, so its not a vast difference. Maybe the airframe couldnt handle in increased power.
It weighed over 200 Kilograms more, that's adding nearly half a ton with two engines then there's the extra weight of the additional wing area and strengthening to take the extra power. The Welkin weighed in at nearly twice as much as the Whirlwind, the improvement in performance was hardly worth the effort when there were other aircraft in service to carry out the role. As Riain said dropping the problematic 24 cylinder engine and making the lightweight Peregrine work would have been a far better option.
What would the Westland whirlwind have become if it had been fitted with Rolls-Royce Merlin engines from the beggining of it's developement?
The wiki data page gives it as 200 pounds, not kilos, more.
And the Welkin was designed as a specific high altitude plane, so that isn't a very useful comparison.
I dont see why it would have been worth developing the Peregrine for just one plane.