OK - wind: Generally conforms to a well-known model with height and is, I guess, 'allowed for' in the performance. You could have thrown-in a temperature inversion. We cannot allow for ALL possible deviations, but icing is fairly easy to determine?
My point again - your 'guaranteed' engine-out performance is required up to 'clean', so include the anti-ice in the calcs. because that affects your max RTOW. When/where/if you turn it on does not matter.