Have you read Rose Cottage or Thornyhold? Those have the same ratio of gorgeous location to extremely mild peril as Stormy Petrel, and that same quality of "Why yes, Mary Stewart, I WOULD like to take a vacation/inherit a cottage in this place!"

They're also all late-career books for her, so I wonder if she just got tired of Peril and settled down to her true loves: charming cottages and plucky heroines with an extensive supply of quotations forever at their fingertips. If so, good on her. They were charming in a very different way than her early books were charming - really not gothic at all, as you say - but once I had adjusted my gothic-o-meter, I find them delightful in a different way.