"Rose and linsie" just doesn't make any kind of sense, so I think it's a corruption for "lily."

There is an idea about, that ballad refrains that deal with the devil or the supernatural had magic words meant to protect the singer. Some people like to sneer at that idea, but it makes sense to me.

For example: Jennifer (gentian) gentle and Rose Marie (Virgin Mary.)
             The dove (Holy Spirit) flies over the mulberry tree.

Before modern breeding programs, there were few spectacular flowers. "Lily" was a general term for any flower that was really large and showy. Gentian, with its vivid blue, was considered a powerful plant, too.

Both the rose and the lily were special to the Virgin Mother. Therefore, in the Cruel Mother ballad, I vote for "lily."

Don't ask me for a bibliography on all this. I read a lot, that's all I can tell you.