Librecad is a substitute of the drawing board
1 Bob already answered
2. Yes, "Flie->import->image" to add scaned image into drawing and draw over it
3. No, .stl are a 3D format and LibreCAD are 2D. To generate .stl files you can use Blender or FreeCAD
4. You are entering into the world of descriptive geometry. A wonderful idiom
To design eyeglass in .stl format you can start drawing it in LibreCAD with acuracy, then open the created dxf file into FreeCAD to convert in 3D and save as .stl. But if you are new to CAD I suggest you start with something simpler like box to store the glasses. CAD power is not to start from scratch a job, is modify an existing job (similar) to create another.
See
http://www.tux-lab.com/This person uses LibreCAD, FreeCAD, Inkscape and Gimp to develop their products.