I think I just figured out how to balance armor pen

All lethality items only apply their armor pen to abilites. LW and its upgrades all apply to AA's only. Black cleaver stays the same. That way ADC's actually have to decide what they are going to focus on for the game and makes them closer to either an early game or hyper carry role. And mixing items makes you more of a mid game champion.

And for the case of Q. LW applies to the AA portion of the damage and lethality applies to the spell part of the damage. This would go for all abilities of similar nature.