QIf there are no followers remaining in my deck, how do I resolve this follower's On Evolve ability?

AIf this follower's On Evolve ability doesn't reveal any followers, simply shuffle the revealed cards and put them back in your deck area.

QIf there is only one follower remaining in my deck, can I still summon it with this follower's On Evolve ability?

AYes. Note that, in that case, you would still reveal all the cards in your deck one by one before doing so.

QIf I have 4 cards on my field, how do I resolve this follower's On Evolve ability?

AFirst, you reveal cards from the top of your deck until you have revealed 2 followers. Then, since you already have 4 cards on your field, you choose one to summon. Finally, you shuffle the other cards you revealed, including the other follower, and put them on the bottom of your deck.