Each portion of the attack gets separate damage reduction from enemy armor,
You probably think "30 armor" means 30 damage is subtracted from each instance of physical damage. But that's not how it works in LoL. Resistances reduce damage by a percentage. Therefore, it doesn't matter whether Xin Q is 100 or 50 + 50 since 77% * 100 = 77% * (50 + 50).
meaning that it deals less than half of its written stats to any potential target - even minions, despite them having 0 armor.
I have no idea how you drew this conclusion. But that's not how it works either. Since minions have no resistances, all attacks apply the equivalent of true-damage.
Check out the wiki. Final Damage is calculated by multiplying Initial Damage by the Damage Mitigation Formula. The Damage Mitigation Formula is defined as
m(r) = 100 / (100 + r)
where
m = mitigation ratio
r = armor or magic_resist
e.g.
m(30) = 100 / (100 + 30)
m(30) = 77%
You can confirm this in game by mousing over you character stats (press the [c] key). A pop-up will display the mitigation percentage if you hover over the resistances.