QIf this follower and 4 One-Tailed Foxes are on my field, and all of my One-Tailed Foxes are destroyed by an ability, does this follower gain +4 attack?

AYes. This follower's "Whenever a Yokai follower you control leaves the field, give this follower +1 attack" will take effect for each One-Tailed Fox that is destroyed.

QWhen does this follower's "if a Yokai follower on your field would deal damage, it deals that much plus 1 instead" ability take effect?

AIt takes effect when a Yokai follower on your field deals attack or combat damage, or damage via abilities. Note that when a follower with 0 or less attack deals attack or combat damage, no damage is dealt at all (it doesn't count as dealing damage).

QIf this follower is on my field, and I attack an enemy leader with a 0-attack One-Tailed Fox, does it deal 1 damage?

ANo. When a follower with 0 or less attack deals attack or combat damage, it doesn't count as dealing damage, and this follower's "if a Yokai follower on your field would deal damage, it deals that much plus 1 instead" only takes effect when damage is dealt.

QIf I have this follower on the field and it has 6 attack. If I attack the enemy leader, and they play Realm of Repose's activated ability with Quick, how much damage does the enemy leader take?

AThey take either 4 damage or 5 damage. When there are multiple abilities modifying damage dealt, the player receiving the damage decides the order the abilities are applied. In this case, depending on the order, the final damage taken can be calculated as follows:

The enemy leader takes 4 damage because they decide to apply this follower's ability to increase the damage dealt by 1 first, so this follower will deal 7 damage before Realm of Repose's ability reduces the damage dealt to their leader to 4.

The enemy leader takes 5 damage because they decide to apply Realm of Repose's ability first, so this follower will deal 6 damage before Realm of Repose's ability reduces the damage to their leader to 4. Then, this follower's ability increases the damage dealt by 1 will take effect so this follower will deal 5 damage.

QIf this follower is on my field, and I attack an enemy Lævateinn Dragon, Defense Form with a 1-attack One-Tailed Fox, how much damage does Lævateinn Dragon, Defense Form take?

AIt takes either 0 damage or 1 damage. When there are multiple abilities modifying damage dealt, the player receiving the damage decides the order the abilities are applied. In this case, depending on the order, the final damage taken can be calculated as follows:

Lævateinn Dragon, Defense Form takes 1 damage because its controller decides to apply the follower's ability to increase the damage dealt by 1 first, so the One-Tailed Fox will deal 2 damage before Lævateinn Dragon, Defense Form's ability reduces the damage dealt by 1.

Lævateinn Dragon, Defense Form takes 0 damage because its controller decides to apply Lævateinn Dragon, Defense Form's ability to reduce damage dealt by 1 first, so the One-Tailed Fox will deal 0 damage before this follower's ability increases the damage dealt by 1. However, this follower's "if a Yokai follower on your field would deal damage, it deals that much plus 1 instead" only takes effect when damage is dealt, and 0 damage does not count as dealing damage. So, the ability does not take effect and the damage taken by Lævateinn Dragon, Defense Form remains 0.

QIf this follower is on my field, and I attack an enemy Lævateinn Dragon, Defense Form with a 3-attack Kasha, how much damage does Lævateinn Dragon, Defense Form take?

AIt takes 3 damage. When there are multiple abilities modifying damage dealt, the player receiving the damage decides the order the abilities are applied.

In this case, whether this follower's ability increases the damage dealt by 1 first, or Lævateinn Dragon, Defense Form's ability reduces damage dealt by 1 first, the damage taken is 3.

QIf this follower is on my field, and I've given a Yokai follower Furtive Fangs's Strike ability, does that Strike ability also deal plus 1 damage?

AYes, it does.

QIf this follower is on my field, and I play a One-Tailed Fox from my EX area, does it cost 2 less?

AYes, it does.

QIf I have 2 copies of this follower on my field, does each Yokai follower I play cost 4 less?

AYes. Each copy's "While this card is on your field, any Yokai follower you play costs 2 less" will trigger.