QIf an opponent returns this follower to hand from my field, does its ability still trigger?

AYes, it does.

QIf I put this follower from my hand back onto my field with its own ability, do I put the evolved form onto my field?

ANo. When this follower is returned to your hand from your field, its ability triggers. The evolved form loses its link to the unevolved form immediately after moving off the field and is put faceup in your evolve deck. Then, you resolve this follower's ability, putting the unevolved version from your hand back onto your field.