QIf I can't select an engaged enemy follower on the field, can I still play this follower's Strike ability to give my leader +2 defense?

ANo, you can't.

QIf an amulet that has become a follower via Maneuver has its attack or defense changed, does it retain those changes if its Maneuver ability is activated next turn?

ANo. When a card's Maneuver ability is activated, its attack and defense become the numbers listed.

QIf an amulet that has become a follower via Maneuver gains Aura, does it retain Aura when it returns to being an amulet at the end of the turn?

AYes, it does.

QWhat does "Maneuver this card" mean?

AManeuver is an ability that turns the card into a follower for that turn. Its attack and defense are listed on the card.

Similar to ordinary followers, a card that has been turned into a follower via Maneuver cannot attack on the turn it is put onto the field, unless it has Rush or Storm.