Well, I really don't think I can agree with you that there was a change in nurse. At the end Mrs. Munro simply refers to "the nurse", and gives no indication that the woman who brought Lucy over is a different person to the woman who originally looked after her.
As you say, Mrs. Munro refers to the nurse as "a faithful Scotch woman who had once been our servant". But the woman Mr. Munro meets is Scottish too: "'What may you be wantin'?' she asked, in a Northern accent." "Northern accent" in contemporary British English suggests an accent from Northern England. But I believe "Northern Britain" used to be used as a term for Scotland, and so "Northern" here means Scottish - the dialect used by the nurse certainly suggests Scotland to me.
The nurse would be from a lower class than Mr. and Mrs. Munro, and she is quite rude to Munro when they first meet (with good reason - she's trying to get rid of him before he sees Lucy). So to Munro, she probably does seem "coarse" - he's viewing her in a negative light. But I'm sure she isn't truly coarse.
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As you say, Mrs. Munro refers to the nurse as "a faithful Scotch woman who had once been our servant". But the woman Mr. Munro meets is Scottish too: "'What may you be wantin'?' she asked, in a Northern accent." "Northern accent" in contemporary British English suggests an accent from Northern England. But I believe "Northern Britain" used to be used as a term for Scotland, and so "Northern" here means Scottish - the dialect used by the nurse certainly suggests Scotland to me.
The nurse would be from a lower class than Mr. and Mrs. Munro, and she is quite rude to Munro when they first meet (with good reason - she's trying to get rid of him before he sees Lucy). So to Munro, she probably does seem "coarse" - he's viewing her in a negative light. But I'm sure she isn't truly coarse.