Ward.
Give your leader +3. If you have 10 max play points, deal 3 damage to each enemy leader.
Whenever this follower takes at least 5 damage, increase your max play points by 1.
QIf this follower's defense is reduced to 0 by damage, will its ability still trigger?
AYes, it will, provided that the damage that reduces its defense to 0 is greater than 5.
QIf I have more than 10 cards in my cemetery, and an opponent deals 3 damage and 2 more damage to this follower on my field with Usurping Spineblade, will my max play points increase by 1?
ANo, it won't. Even though Usurping Spineblade does a total of 5 damage, it does so in two instances of damage, and this follower's ability only applies to a single instance of damage.