I could barely hear the i. I suppose that happens with natural speech.
I suppect it is the way it flows, to me "Chan eil i" sounds like
Maybe cause the start of "eil" is a vowel and "i" is a vowel, like in Italian and French
Why is it not 'he is not rough'? Is it because that would be 'chan eil i a garbh'?
The i is feminine. If it was masculine, e would be used instead.
It's referring to the weather, and weather is feminine in Gaelic. But in English the weather is neuter (in common with all unsexed nouns) so we use it, so the translation uses "it".
The Gaelic being used does not sound like that commonly spoken in Scotland!
The dialects are said to vary a great deal across the region and islands -- and even from speaker to speaker.