How long do the coins take to mature? The app says there's a treshold of 10 ETN. I got 20 ETN now, but it's still pending and hasn't been transfered to my account.

Actually that's not headless. The commandline wallet is missing. In order to stake you need to run the wallet, and the graphical qt wallet has to be displayed on a XServer (or similiar). A commandline wallet could just run in a screen session without any display attached.

Is there a headless wallet for Linux? Source on Github appears to be daemon and qt wallet only.