QIf there are 4 cards on my field, do I still have to select both a non-Abysscraft follower that costs 5 or less and a non-Abysscraft follower that costs 3 or less in my cemetery?

AYes. You must have both in your cemetery to play this follower's ability. Then, since you have 4 cards on your field, you may summon either of your choosing.

QCan I play this follower's On Evolve ability if I don't have a non-Abysscraft follower that costs 5 or less or a non-Abysscraft follower that costs 3 or less in my cemetery?

ANo. You must have both in your cemetery to play this follower's ability.