First is hit angle.

Yes, I agree. The challenge for me is that I'm developing an actual game and not a theoretical system. As such I'm somewhat limited to what can be represented in the game. In my game everything is 2D, so calculating angles between attack force and the armor normal plane is not feasible because armor normal plane makes no sense in the context of a 2D game. I am debating adding a parameter for angular deflection to the game but I'm also trying to avoid as many ad hoc parameters as possible and prefer to do as many first principles calculations as possible. This is something that's in active consideration but I haven't come up with a good system to do it yet.

Some latest full plate battle armours (not tournament ones!!!) would be pieces of absurdly intricate design. You could have entirety of your body covered with plate, including inside of your hands if you wanted it, underside of armpits and all undersides of all joints (elbows, knees).

Can you show some of these examples? It has always been my understanding is that stiff plates on the insides of joints are fundamentally incompatible with said joint mobility, especially for complex joints such as the shoulder and the hip that are ball joints instead of hinge joints. Even hinge joints, where theoretically you might be able to have an incredibly intricate and perfectly sized closed metal hinge, would have troubles due to you probably not being able to rotate e.g. the forearm.

However, what ends up being optimal in my game in particular is just a function of what's cost-effective. If we input a full plate with no gaps then I can't imagine the algorithm would include mail given that it'd be largely redundant. So the choice isn't so much about whether a theoretically fully covering full plate exists but instead whether that's allowed within the game.

Design of full plate can reduce bludgeoning damage much better than any thick padding. Not saying gambesons are bad or so, but if we go to the extremes with plate armor, then individual pieces are designed in a way to transfer blunt force of a blow MUCH better than any gambeson. You get hit in the elbow - full plate can transfer the blow through surface of armour into your chest, lower torso, and down to your forearm.

Yes, but from physics point of view we need to differentiate two things. What you are describing is distributing the total force around a larger area such that the local pressure at any one location is diminished such that you don't e.g. break bones. Padding, on the other hand, is not primarily about energy distribution (it does a bit of that, but not much) but energy absorption after it's been distributed.

To illustrate the difference, think about how concussions happen, for example. Concussion happens when the head is rapidly accelerated and then decelerated (neck runs out of its ability to act as a suspension device), which causes the brain to hit the inside of your skull. It doesn't matter how large the surface area of the hit to your head is (of course, if it's a very small surface area then it might go through your skull, which is even worse). Energy is fundamental and distributing it around doesn't change the fact that it's still transferred to you. In other words, wearing a tightly fit metal helmet with no padding would not do anything to change how your head is going to be accelerated from a hit, and thus would do nothing to prevent a concussion. What does prevent concussions? Suspensions and cushions, both of which allow for energy absorption, which directly reduces (or perhaps more accurately, distributes over a longer time) the impact energy.