Poison And Armor Break General Questions Too

So, if I use Poison Arrow and break the Armor of the Enemy, he now has reduced Armor which means my Poison Arrow does more Physical Damage to apply a stronger Poison?

Also what I don't understand is Poison Duration.

Let's say I hit for 1k Poison. Default duration is 2 seconds. So the Poison will deal 1k in 2 seconds.

Now let's say I increase durations of my Poisons to 4 seconds. I still hit for the same 1k, that now means the Poison will deal the exact same 1k Damage over 4 seconds now, and not 2 .. which means I just weakend my Poison by increasing Duration.

If that is true, what's the point then of Stacking Poison to example 8 Stacks for Pathfinder?