League-of-legends – Does exhausting a champion reduce the damage done by its pets

Lets break this out piece by piece.

• Does armour reduce it, the same it would for auto attacks?

Armor does in fact reduce the damage you take from normal towers. The base tower however deals true damage.

• Do items like thornmail and skills like Rammus' curl reflect damage?

Simple answer: No. Most abilities do not deal damage to towers either. Exceptions include Heimerdinger's turrets and Alistar's passive.

• Do skills like Maoki's ultimate (-15% damage in the aoe) or Alistair's ultimate (-75% damage on self) affect the turret damage?

Maoki's ultimate does not reduce damage taken from turrets but Alistar's ultimate does.

• Does the last defensive mastery negate 4% of the damage?

I believe so for the following reasons. It says it reduces "All damage" doesnt exclude towers specifically (the way Maoki's ultimate description does).

Hope that helps.

Mordekaiser received a rework since the previous answer was posted, which completely changed the way his ultimate works. Previously, it applied a damage over time effect that would create a clone pet of the target for Mordekaiser to command if the target died under its effects, and this could target and affect Shaco's clone.

The ultimate's description now reads:

R - Realm of Death - Cast time: 0.75sec

Mordekaiser slows the target enemy champion by 75% during the cast time, after which they're banished together with Mordekaiser to the Death Realm for 7 seconds.

Units between realms see each other as spirits, considering each other dead and negating any interactions between the two. Only the target and Mordekaiser will enter the realm; other champions cannot follow them. Everything that occurs inside the Death Realm is hidden, and all still inside are killed at the end.

For the same duration, Mordekaiser also consumes the target's soul, reducing their current attack damage, ability power, total attack speed, maximum health, armor, magic resist, and size by 10%, in addition to gaining them for himself. If the target dies while inside, Mordekaiser keeps their partial stats until they respawn.

If one of the two affected champions leave the Death Realm, because of having died or having it dispelled, the other will do so as well, but the stats will remain stolen for the full duration.

Shaco's clone, while considered a "champion" for the purposes of most abilities, is not affected by Mordekaiser's ultimate ability now. Mordekaiser will perform the cast animation, but both he, and the clone, will not teleport to the Death Realm.

Here's a gif, taken from this video, highlighting what happens when this interaction occurs:

Not pictured is that this does put Mordekaiser's ultimate on cooldown, so it is not beneficial whatsoever for Mordekaiser to use his ultimate on a Shaco clone.