That doesn't sound like too bad a spot as far as non ideal positions go.
Speaking of omni-directional speakers, I ran my AV-1RS as "mains" for a couple weeks when I was breaking in, that actually didn't sound too bad, although, I could kind of tell that the music was coming from up there.
I would do the N3 and see how you like its bass response before deciding on a subwoofer.