Thank you so much for your helpful suggestions.
For some reason, my program (X1, Build 11.5.4.17) doesn't seem to work the same way. For example, when editing my customized toolbar, I couldn't find the "break line" icon anywhere. (Even if it was greyed out when not available, I figured at least if I had it in my toolbar I could find it much easier.) Nor could I find it under any of the software's menus or submenus (not even in CAD mode; nor when a line is selected).
David: Thank you for your helpful tutorial. Unfortunately, when I grab the corner pieces of any roof plane, they don't move at all (even if I hold down the "Control" key). The only way I was able to change the roof line is to grab the diamond-shaped pieces along the straight edge, and those are limited to moving in parallel lines. For some unknown reason, I cannot manipulate them the same way you do in your video.
Either way, I'm thrilled that the shortcut buttons (number 3 and "make parallel" tool) were able to successfully accomplish the task.
Thank you again to everybody who replied!