Armor is now 100/(100+max(0, armor-penetration)), and mitigation is a flat damage reduction (eg 8 mitigation means all instances of damage deal 8 less while shields are still up).

As for whether it will stay, I suspect it will, but I've not seen anything to confirm or deny that, so I'm just assuming the status quo. The devs still have several dials they can tweak to control interactions (eg missiles independently get reduced by shield mitigation, making the Marza missile bank much weaker than it looks on paper, but if TEC perform poorly against Advent, they might reduce the salvo size, spreading the same total damage across fewer missiles).

Correct.

In the case of the Marza's missile battery's .weapon file, we have the below (snipped for brevity)...

The total volley does 273, but because there are 12 missiles in the burst, each one only does 22.8. Against a capital ship with 8 shield mitigation (the TEC standard), that's a total of 96 less damage, a 35% reduction to the volley.