After reading everything you wrote I immediately thought..damn that must be a really horrible website navigation nightmare. So I went to the website, searched for an artist and dragged it. Then I played it. Then ..(cringe)..I did it....again. In summary, as of the moment of this post, the small scale nuclear holocaust of complexity you have described apparently lay dormant. My experience was actually kind of boring.
Edit*
I see what is happening. There are a couple of paths users can take based on if they click or drag items, and I just so happened to take the smoothest one.
Click to add is faster when adding to the playlist. It's just the addition of the small plus to the play symbol isn't immediately obvious if your mouse is hovering over it. I like it better than click to drag. But drag is surely best for first encounter.
Edit*
I see what is happening. There are a couple of paths users can take based on if they click or drag items, and I just so happened to take the smoothest one.