Linking polygons and off-screen rendering

I’m very new to OpenGl so please excuse the banality of my question.

I have a number of structure to be displayed, each consisting of a number of polygons and each polygon comprising a number of vertices. I would like to display individual polygons using GL_LINE_LOOP but I want to link all polygons within a structure so that the complete contour (mesh) of the structure is produced.
i) Is there an easy way to do this in OpenGL?
ii) Some of the structures are split, i.e. two or more polygons in a single plane (z). How can I display this as a mesh

iii) Finally I would like to dump the frame buffer to a file, or better still into another C++ object in order to perform further operations. Ideally I would like to write a binary mask representing the structure having n*m pixels (viewport), where a pixel value of 0 is outside structure and 1 is inside ( I want to form an aperture using extremities of the structure for each scanline in the viewport).