Cemetery Reaper grants +1/+1 to all other Zombie creatures you control, not just the ones its ability puts onto the battlefield.

The creature card in a graveyard is exiled as part of the activated ability's effect, not as a cost. If that card has left the graveyard by the time the ability would resolve, the ability doesn't resolve and you don't get a Zombie token.

A "creature card" is any card with the type creature, even if it has other types such as artifact, enchantment, or land. Older cards of type summon are also creature cards.