Shadowborn Demon’s enters-the-battlefield ability is mandatory. If you control the only non-Demon creature, you must choose it as the target.

The last ability checks whether you have fewer than six creature cards in your graveyard when it would trigger. If you have six or more, it won’t trigger at all. The ability will check again when it tries to resolve. If at that time you have six or more, the ability won’t do anything.