I recognise that the software may have been designed to work as such, but:

1. I expect a song to start playing after double clicking on it.
2. When I am inside an album, and double click on a song, I expect it to start playing, and switch to the next song when finished.
3. When I am in the list view of all songs, see point 2

Why not switch to this behaviour by default, and provide an option to switch if people desire otherwise?

The + sign could be placed somewhere more convenient, maybe to the left side of the song title, and always visible to show people that this button is for adding songs to the playlist?