"Cò thusa? Is mise Mòrag."
Translation:Who are you? I am Morag.
If anyone is wondering, that is very old grammar. Is milis Mòrag would be 'Mòrag is sweet'. The gu at the beginning means 'that' (relative) as in '[she said] that Mòrag is sweet' or something, but I cannot figure out what it is doing at the beginning of the sentence. Perhaps someone is thinking 'that Mòrag is sweet'?
Edit: I found the words, with translation, here
It must be that the singer is thinking 'that Mòrag is sweet'. He is is also thinking that she would be much sweeter if Tormod were 'cold under the turf'. How charming..
Yes. It is a good question as it is often not explained what is going on. Worse, you see contradictory explanations. But all we need to know is that this one word is disappears in questions and negatives (and other situations that use the question form that you will learn about later). So all your examples are correct, and we also have
An tusa? Is it you?
Cha mhise It's not me.
Nach Seòras? Isn't it George?