< you have to hold down on it for a moment.
Haaaaaaaaaaaaaaaaaaa never think of that! :)

<You mean vertical as in the z direction up from the plane?
Yes :)

< You can create such a one by doing a second guideline snapping the first point on to the start of the original guide line
Sorry don't find yet this in the 3D view :(

It's not a problem else now I can draw another vertical in the Front or Right view and move it with your curious "cline" cursor :)
(after press it a moment of course:)

All the rest seems make sens :)