I may be able to help with acouple of these questions
Key equivalent for left mouse would be the "ins" key, it sets a point at the current cursor position, conversely the period key is equal to right-mouse click, its sets a point on top of the nearest existing point in the drawing, comma moves the cursor to the nearest point without setting a new point, all have their uses!!
To extrude your triangle along the top of your box, select the triangle by one of its base corners - (left-click near the corner), then from the menu "draw, extrude", then set points using right-mouse or period on the corresponding front and rear corners of the box, and hit enter, and bob's your uncle, I mean there's your roof.
As far as I'm concerned, and quoting from several previous posts by myself, Designcad's greatest feature is you just open the program and start drawing, no setup required. The right-click snap to existing is close behind.
The thing about computer drawing is, I think it's always best to draw exactly to true size from the start, you need to enter some kind of dimension for everything, it may as well be the correct one. That way, you can see right away if something works and if it will look right, because it looks exactly as it will in the real thing.