forty years ago, I picked up a children's book on the architecture of American homes. One of the homes was a Dutch house from the 17th Century, it was made of brick, and on one end of the house, close to the ridge of the roof, a brick was missing.

Why? It was traditional to leave that brick out so that the souls of the dying could escape to heaven through the opening.

I doubt if the Dutch were the only 17-C people who thought about souls in that way. So I suspect that the light that the soldier saw, 'springing from her clothes' as she lay dying, was her soul, leaving her body.

(Since she's dying, the family allows him to enter their house and stand at her bedroom door, "where he had never been before".)

In the third verse, the father cries, "My daughter is dead...", but it's not news to the lover. He knows it already.   
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FWIW, I searched for 'Flandyke Shore' on the Bodleian Broadside site, and they said they didn't have it. What's the source that quoted Mrs. Notley?