Sachs- Huret derailleur (re)installation
Here's an image of your derailleur showing the return spring attached to the slot furthest from the top of the cage
https://c1.staticflickr.com/9/8111/8602860708_0e4e4a50ef_b.jpg

To answer your second question, seems reasonable to replace the double nut on the outside of the fork end with a single nut.