In this case (3) i would like to know what a healthy way to deal with that?
In principle you can use one key for multiple devices and multiple keys on one device, so you're pretty much free in designing your setup.
Easiest way would be to have one key (preferably with password) and distribute it accross your devices. As long as you trust all of your devices this is somewhat reasonable.
You can also use one key per device. In this case you're able to remove a key if you loose the device/key-file and password. Since you can use any key to add/remove others (at least as long as you don't implement complex permission control) there is no real need for special control stuff. You can of course add an additional key, put it on a stick and lock it away just to be sure to have access.
I mostly use a mix - I got a backup key locked away and a HSM (something like a SSH-key on a USB-stick) which I carry arround and share across devices.