Okay, well, first let me say that I'm not sure how to answer your question, but I would like to think through it a bit myself. The electrical potential energy at the beginning should be equal to the sum of the kinetic energies of the spheres at the end. I think conservation of momentum still...
The answers are in the formulas you've posted. If lever arm = r sin theta, and torque = force * r sin theta, then torque = force * lever arm. Since you know torque and force, you can solve for lever arm.