Regarding songs about natural beauty and in praise of the landscape, versus dramatic events, violence and death.

Have a quick browse through traditional songs, and those that have a narrative structure heavily feature what might be considered lurid, or "gratuitous" violence.

It's not in any way unusual. What is unusual in the oldest traditional songs is any notion that the natural world is beautiful, or worthy of praise. From perhaps the 18th century onward, the modern poetic viewpoint emerges, and it's from then that we get lots of idyllic "isn't Nature lovely?" songs and poems.

So I think it highly likely that the versions of "Gadie" which go on about the fragrant heath, yellow whins and bosky linns, etc, are later "improvements" on the old song about drowning, murder, and burial...