QIf I play this spell, then put a follower onto my field with Death's Breath or some other ability, do I give it +1/+1?

ANo. This spell's ability only triggers when you put a follower onto your field by playing it.

QIf I play 2 copies of this spell, then put a follower onto my field by playing it from hand, do I give it +1/+1 twice?

AYes, you do.