I don't see how that changes anything. Post the single with a "Available to buy on <some date>" notice. When it's officially released, take down the notice an

I don't see how that changes anything. Post the single with a "Available to buy on <some date>" notice. When it's officially released, take down the notice and put up "Buy now" buttons. And there's no reason they have to make every song available for sale or every song that's for sale available for streaming.