I did some more research and thinking, and my final theory is that the shackle angle is used to manipulate how the weight that the leaf springs support is distributed in relation to the eye. If you have an angle where the shackle is closer to that "B" line, you have more of the weight pushing down and twisting the spring clock-wise, This would make it "softer" but would really just make the spring compress further. If the shackle is closer in relation to "A", the weight will still be down and wanting to flatten the spring, but the torque on the spring eye will be counter-clockwise which will promote the spring to arch instead to flatten or compress along with the down force.
In terms, of how the length of the shackle affects the up travel, since all of the weight is being pushed up higher due to the length of the shackle. Your potential energy will increase with that height increase in relation to the leaf spring, which means the downward velocity all of the weight supported by the leaf spring (rear of the car) will increase. The energy is conserved therefore, the amount of displacement in the leaf spring will increase because energy from the vehicle is transferred to the spring in terms of elastic potential energy. More EPE equals more displacement in the spring, which is the additonal "flex" given. Momentum is also in play here and the vehicle will have more momentum because it has more kinetic energy, which is transferred to the spring. This work together though and it dives deeper than I desire. Feel free to chime in or point out any mistakes in my thinking!
Also, I hope everyone has a Happy Thanksgiving!