I’m not sure what reference geometry you are referring to. The sides of the box make the reference geometry to aid in locating and correctly orienting the gable ends of the roof. Note that with some practice you shouldn’t need the box and I only suggested it as a simple guide to get you on your way.
I used the Line tool to do the drawing. I used the Protractor tool to set out some angled guidelines from the ends of the horizontal line. I set them to match the pitch of the roof.
It looks like you managed to get a reasonable representation of the roof. If you zoom in close to where the two ridges meet you’ll see one ridge is very slightly higher than the other. If this represents the way they actually are or will be, that’s fine but if not. It’s probably worth dealing with.
That’s an interesting part of a roof like this. The exact solution depends on what is supposed to happen with the building underneath. Note that the roof has no thickness as it is. Maybe think about how the roof would really be built and model that. Are there fascia and rake boards around the edges? Should there be a wide soffit under that corner? Or is it open to the rafters? All those sorts of details come into play.
Yes you can. If the line you want to draw is parallel to an axis line, just watch for the colored trailing line. If you want to go perpendicular to an existing edge start the edge, then hover over the existing edge for a couple of seconds and move the cursor away watching for the magenta inference color and the Perpendicular message. Either Perpendicular to the edge or to the face.
You might want to have a look at these on inferencing.