Nope, the who in a great horned is a distinctive who with a deep tonal quality. This was definitely more of a child's slide whistle quality, more of a toot. I'm thinking it was a northern saw-whet as it most closely matches that tonal quality. Here is a link that might be useful: Listen to

Barn Swallows for sure. I responded to your other post; once Barn Swallows find a good place to nest, they tend to return. Generally taking down any existing nest will just have them build another. Is that a nest box? If so, you may have better luck removing that....but they probably will just build new.

Yep, colours work out. Today I had the male show up too. Both birds are walking very diligently through my flower bed in search of seeds. They make quite a different sound when startled.

There are half a dozen subspecies of American robin. The ones from Newfoundland are sometimes called "black-backed robins", and in addition to being much darker than the more common subspecies are somewhat larger. Birds from the rain forests of the pacific Northwest are also very dark. Probably what you saw was an adult male bird from one of those subspecies.