Can someone briefly explain or point me to an explanation of why the firing pin block spring, and the (precocked?) hammer spring affect the trigger pull?
The firing pin block is moved out of position as part of the trigger press. As such, the heavier the spring, the heavier the trigger pull... in particular, the heavier the takeup.
The mainspring still gets compressed a tiny bit at the break of the trigger stroke when the sear is released. As such, the heavier the spring, the heavier the trigger pull... in particular, the heavier the break.
Neither makes a huge difference, but in my experience the two combined help offset the added weight from the (desirable to me, YMMV) heavier trigger return spring.