I don't get the grammar structure of "cold it must be." How come the noun and the pronoun could be placed in a row?
Äidinkielenään puhuvan vastaus
Rebecca
This phrase is a bit tricky. I do not believe this sentence is grammatically correct. It functions more like an expression. Expressions like this tend to not focus strongly on word order or grammar. As an American English speaker, I have never heard this expression used.