Well here I go again with another sketch of the mechanism. It's not finished, but shows the most important parts of the new layout.
Because I can't find any leafsprings, I now plan to use some coilsprings I still have.
In this new design the sinew is attached to the outer tips of two rigid throwing arms made of metal (blue parts). Every arm is fixed to the crossbow frame by a bolt (circle with the cross). The bolts alow the arms to rotate freely. The red connection rings attach the arms to the green coilspring. When the sinew is pulled back the arms move backward and extend the coilspring, storing a great amount of energy in it. The sinew is then held by the release mechanism, until the trigger of the crossbow is pulled.
The sketch only shows the "nut" of this mechanism [link]
For a sketch of the release system look here [link]
edit: added the loading mechanism. It's basically a crank used to pull the sinew. As always the cross indicates the fulcrum. And I removed the stupid nut because it looked ridiculous!