My mother used a piece of wood in the foot pedal to slow the machine when she taught us to sew. My machine has two speeds, which I have used the same way.
Some machines allow you to insert two needles into the shank.
With the machine you have, are you sure it was designed to handle twin needles? Or is there a width setting you are exceeding to allow it to work with them?
Charging your neighbor for breakage sounds like a good idea.